Pressure vs powder/chamber volume and Barrel Life

[RAG]

I have a couple theories I've been formulating, as I slowly but surely burn through barrels on the dog towns. First, powder volume, or degree of overbore, is a much bigger detriment to barrel life than running high pressures. To use an example, you can take a 223AI and run it really hard (high pressures)...let's say 3,700 fps with a 53gn bullet and still get 5,000 rounds before accuracy falls off too much. Or, it seems to me, you can step up to a 22BR and run it at relatively moderate pressures to achieve this same velocities with same bullet, yet you'd be lucky to see 2,500 rounds before accuracy really starts to fall off. I've certainly not done any scientific double-blind studies here, just seems to me, you can run a moderate volume cartridge really hot and still get great barrel life...yet I've not seen overbore cartridges like a 22BR or 22-250 last long, even when pressures were kept low.

Second theory, I also believe there is a threshold you cross, in regards to case volume (relative to bore diameter) where increasing case volume past that threshold increases barrel erosion exponentially. In other words, to get an extra 100-150fps, you might but barrel life in half, or even 1/3rd. Take the mighty 6BR...I've burned out several barrels pushing 55/58 grain bullets HARD, yet I still get about 6,000 rounds of very accurate barrel life typically. But step up into a DASHER and run these same bullets an extra 150fps, and you'll likely get about 1/3d the barrel life. Big price to pay for crossing that 6BR capacity threshold. For the 22 cal, I suspect this threshold is somewhere north of 223AI capacity...go much above that, and watch out!

 

[Forum Boss]

I don’t agree with your assertion at all regarding .223 AI high pressure barrel Life vs. 22BR at moderate pressures. Think you have to look at multitude of factors including firing rate, barrel type.

Also: "But step up into a DASHER and run these same bullets an extra 150fps, and you'll likely get about 1/3d the barrel life." I dispute that. This has NOT been my experience AT ALL with a 6BRDX, and I doubt 95% of Dasher shooters would agree with you at all. Yes, there may be increased throat wear (particularly for short-necked Dasher compared to a BRA) and some reduced barrel life. But the notion of "1/3 the barrel life" is very implausible, based on my conversations with dozens of skilled Dasher shooters.

Some of the statements in the original post simply don't make sense at all -- as if this whole thread was started as some kind of prank.

 

[Uthink Uknow]

I think you are taking too much time between shots to think about barrel life.
When you think of barrel life consider this: a bullet is in the barrel for a microsecond and a barrel shows signs of wear at 2500 shots so a barrel is only good for 2500 microseconds.

 

[JEFFPPC]

I think you are taking too much time between shots to think about barrel life.
When you think of barrel life consider this: a bullet is in the barrel for a microsecond and a barrel shows signs of wear at 2500 shots so a barrel is only good for 2500 microseconds.

Great answer, lmao.

 

[357Mag]

I have a couple theories I've been formulating, as I slowly but surely burn through barrels on the dog towns. First, powder volume, or degree of overbore, is a much bigger detriment to barrel life than running high pressures. To use an example, you can take a 223AI and run it really hard (high pressures)...let's say 3,700 fps with a 53gn bullet and still get 5,000 rounds before accuracy falls off too much. Or, it seems to me, you can step up to a 22BR and run it at relatively moderate pressures to achieve this same velocities with same bullet, yet you'd be lucky to see 2,500 rounds before accuracy really starts to fall off. I've certainly not done any scientific double-blind studies here, just seems to me, you can run a moderate volume cartridge really hot and still get great barrel life...yet I've not seen overbore cartridges like a 22BR or 22-250 last long, even when pressures were kept low.

Second theory, I also believe there is a threshold you cross, in regards to case volume (relative to bore diameter) where increasing case volume past that threshold increases barrel erosion exponentially. In other words, to get an extra 100-150fps, you might but barrel life in half, or even 1/3rd. Take the mighty 6BR...I've burned out several barrels pushing 55/58 grain bullets HARD, yet I still get about 6,000 rounds of very accurate barrel life typically. But step up into a DASHER and run these same bullets an extra 150fps, and you'll likely get about 1/3d the barrel life. Big price to pay for crossing that 6BR capacity threshold. For the 22 cal, I suspect this threshold is somewhere north of 223AI capacity...go much above that, and watch out!

RAG -

Howdy !

Not in any necessary order.....

Renowned ballistician Homer Powely has stated:
" High pressure is high temperature, and therefore; fast erosion. It is as simple as that ".

" The ballistic efficiency inside a gun is determined by the pressure. Therefore, we can say that efficiency in any gun is going to be limited by the limit of pressure which can be used ".

FWIW -
There was a rifle/ cartridge " overbore " chart that was circulating around; some years back.
In .224" calibre, the chart showed " overbore " capacity began right @ the .220 Swift capacity level. By extension, then.... a .22BR and a .22-250 would not be classically
" overbore ".

I have owned / shot a .22BR benchrest rifle, and numerous .22-250s.
Additionally, I was well acquainted w/ the loads and terminal effects for both my best friend's .220 Swift; and my wildcat " .22-35 Remington ". The latter has case capacity between that of a stock .22-250 and a .22-250AI. For use on groundhog, we shot the Hornady 55SX w/ all the above cases.

Based on my .224" calibre experiences ( use on groundhog and paper targets ), I am inclined to agree with the overbore threshold listed by the chart I mentioned above.


With regards,
357Mag

 

[357Mag]

I have a couple theories I've been formulating, as I slowly but surely burn through barrels on the dog towns. First, powder volume, or degree of overbore, is a much bigger detriment to barrel life than running high pressures. To use an example, you can take a 223AI and run it really hard (high pressures)...let's say 3,700 fps with a 53gn bullet and still get 5,000 rounds before accuracy falls off too much. Or, it seems to me, you can step up to a 22BR and run it at relatively moderate pressures to achieve this same velocities with same bullet, yet you'd be lucky to see 2,500 rounds before accuracy really starts to fall off. I've certainly not done any scientific double-blind studies here, just seems to me, you can run a moderate volume cartridge really hot and still get great barrel life...yet I've not seen overbore cartridges like a 22BR or 22-250 last long, even when pressures were kept low.

Second theory, I also believe there is a threshold you cross, in regards to case volume (relative to bore diameter) where increasing case volume past that threshold increases barrel erosion exponentially. In other words, to get an extra 100-150fps, you might but barrel life in half, or even 1/3rd. Take the mighty 6BR...I've burned out several barrels pushing 55/58 grain bullets HARD, yet I still get about 6,000 rounds of very accurate barrel life typically. But step up into a DASHER and run these same bullets an extra 150fps, and you'll likely get about 1/3d the barrel life. Big price to pay for crossing that 6BR capacity threshold. For the 22 cal, I suspect this threshold is somewhere north of 223AI capacity...go much above that, and watch out!

RAG -

Howdy, again !

Your comment about " chamber volumne " has opened the door for a discussion about rifle' " expansion ratio "......

Powley told us:
" ....expansion ratio determines how much of the chemical energy is converted into kinetic energy . "

" Th

 

[Bottom Fodder]

Correct me if wrong:
does “ over bore” large case small bore case cause throat erosion and high speed bullets cause barrel wear? but with either one the barrel can be toast

 

[357Mag]

RAG -

OOoops.... transmitted in-error.

Let me try, again......

When talking about total capacity of the gun, that would include the chamber volume plus the bore volumne.

"The expansion ratio is simply the ratio of the total volumn of the gun to the case capacity."
" It is the number of times the gas will expand by the time the base of the bullets reaches the muzzle ".

" Lower expansion ratio guns are lower in efficiency than higher expansion ratio guns ".

Even shooters that claim there's no such thing as "overbore " oughta' still agree that an
" overbore " chambering will have a lower expansion ratio; based on Powley's assertions.

Here's the plan:
For a given calibre and chamber volumne, the way to change expansion ratio is by changing the barrel length.

For a given calibre and barrel length, the way to change expansion ratio is by changing the chamber volumne.



With regards,
357Mag

 

[357Mag]

BRX6 -

Howdy !

More from Homer Powley:
" Barrel life does not depend upon the velocity of loads, as such. "
" IF high velocity has been obtained by running high pressure, then the cause is still
high pressure; and not velocity ".

Powder combustion " turbulence point " is something that has been put forward by former Savage Arms engineer Robert Greenleaf. The chambering's " turbulence point " might be notionally described by viewing the case is 2D side view.... extending the shoulder angle lines to their ultimate convergence point... and then noting whether that convergence point lies within the case' neck area of the chamber. Chambering's that have been noted as being hard on barrel life can sometimes be found to have the " turbulence point " outside the case' neck area; and into the freebore area or leade'. The .243Win is one example of this phenomenon ( IMHO ).


With regards,
357Mag

 

[Laurie]

For a given calibre and chamber volumne, the way to change expansion ratio is by changing the barrel length.

For a given calibre and barrel length, the way to change expansion ratio is by changing the chamber volumne.

There is general confusion as to the meaning of the term 'Expansion ratio'. As I understand it, the first of your definitions is the strict (ballistician's) definition of the term. As you note, that introduces a firearm related variable that is quite unrelated to the cartridge design.

I have seen an unofficial term in use for the latter, the 'Effective Expansion Ratio' that is purely cartridge design related. For example how far does the bullet travel to double the volume of the combustion chamber. In a 45-70, that's a much shorter distance than in 243 Winchester.

The 'Over bore' [capacity] type metric is the ratio of case capacity to bore diameter. The easiest (and most commonly used) way to calculate this is fireformed case 'overflow water capacity' in grains weight as used in QuickLOAD, GRT etc divided by the bore size area in square inches. There is no formal / official definition of where the 'over bore capacity' boundary lies, partially because the availability of very slow burning powders keeps increasing. A previously over-bore cartridge design that gave little or no additional MV for the additional powder charge weight over lower case capacity models in P O Ackley's days when he considered IMR-4350 and 4831 as exceptionally slow-burning magnum powders is much less so today with Retumbo, Re33, 50BMG powders etc available to the handloader. Nevertheless the 7mm Remington Magnum with a case capacity to bore area ratio of 1300 was long regarded as the borderline, and is as good a boundary as any even today. (Today's 7WSM which many users define as a finicky number to load for precision use is also 1300 and can wear barrels out very quickly if loaded up to its maximum. The 300WSM using the same basic case under a bigger bore at c. 1,080 is a much better mannered beast, regularly noted for being easy to tune with wide accuracy nodes. I shoot the 300 SAUM which with c.987 is a real sweetie, very easy to tune indeed. 308 Win at the 750 mark is hugely powder and load tolerant and gives great barrel life if loads / pressures / firing rates aren't abused.)

This ratio is one of the major determinants of the optimal powder burning rate required as well as barrel life. The higher the value, the slower-burning the powder needed. Bullet weight is also a major factor too, as nearly everyone knows. More open to argument, heavier bullets also wear barrels out faster too in that more mass = greater inertia = the bullet spends longer time in the throat/leade area under maximum pressure, and ...............

Renowned ballistician Homer Powely has stated:
" High pressure is high temperature, and therefore; fast erosion. It is as simple as that ".

extreme heat for longer in the bit of the barrel that determines its life.

Another new factor which has the same effect is Swiss chemical company Nitrochemie Wimmis AG with its patented 'EI' powders as seen in a few Alliant Reloder grades and some of the Reload Swiss grades we have in Europe. 'EI' infuses the deterrent chemicals into the kernels under the surfaces which therefore work for more of the burn than traditional simple surface coatings. This extends the controlled burn period in time / bullet travel and allows the peak of the pressure curve to be longer/flatter. As with heavier bullets it means peak pressure lasts for a longer time, but also now bullet travel too. As this increases the area under the pressure curve it improves MVs - drastically in some cases such as RS60/Re17 in the 6XC and 284 Win with match loads getting boosts of a claimed 200 fps in some combinations. Extended peak pressure = extended period of peak heat energy = increased wear as people who maxed loads / pressures out with this powder soon discovered especially in BR and US F-Class with string-firing and rapid heat build-ups.

Years back somebody on this forum posted jpegs of an article written by a Finnish ordnance officer of ES and group size growth vs round count in the NRA US Rifleman magazine using the Finnish 7.62X53R service rifle with a standard but selected barrel. 7.62X53/54R has a bore capacity ratio of c. 850, or 100 higher than 308 Win, but its nominal maximum peak chamber pressure is 390 MPa (56,565 psi) under modern CIP methods and standards. I suspect the service cartridges used in these Finnish Army tests were somewhat, possibly considerably, lower pressure. ES values, likewise groups, only started to increase slightly after the barrel was at 11,000 rounds from new, likewise MVs started slowly dropping. The tests were discontinued at over 13,000 rounds with the rifle still shooting well, but the testers worn out. I don't know what effect the qualities / hardness of Finnish chrome-moly barrel steel had on the outcomes compared to the relatively soft 'stainless' we use in match barrels, but the relatively low case capacity to bore ratio and chamber pressure values must play a large part in providing this level of barrel life. 65Grendel forum members say their AR-15 6.5 Grendel (capacity value 665 based on Lapua in my rifle's chamber); SAAMI 52,000 psi max pressure) barrels (presumably stainless) last 9-11,000 rounds - and I don't suppose most such shooters shoot strings of five rounds slowly, then let barrels cool before resuming! Hearsay I know, but I wouldn't disbelieve it. Scandi 6.5X55 shooters with their Sauer STR200s using modern chrome-moly steel barrels and mainly factory ammo which I presume sticks to the old Swedish Mauser max pressure around 50,000 psi (less?) report excellent barrel life, far better than many people I know using hot 6.5 Creedmoor handloads with heavier bullets in stainless barrels (1056 capacity ratio vs 975 for 6.5X55 and 6.5 Creedmoor).

So whichever way you play it, military scientific testing or hearsay, any of the individual metrics of high chamber pressures, high bore capacity ratio values, high rate of fire reduce barrel life all other things staying the same, but in combination will severely reduce it.

 

[RAG]

I don’t agree with your assertion at all regarding .223 AI high pressure barrel Life vs. 22BR at moderate pressures. Think you have to look at multitude of factors including firing rate, barrel type.

Also: "But step up into a DASHER and run these same bullets an extra 150fps, and you'll likely get about 1/3d the barrel life." I dispute that. This has NOT been my experience AT ALL with a 6BRDX, and I doubt 95% of Dasher shooters would agree with you at all. Yes, there may be increased throat wear (particularly for short-necked Dasher compared to a BRA) and some reduced barrel life. But the notion of "1/3 the barrel life" is very implausible, based on my conversations with dozens of skilled Dasher shooters.

Some of the statements in the original post simply don't make sense at all -- as if this whole thread was started as some kind of prank.

Nope. Not a prank. I've been burning out barrels in high volume shooting for a while. While I shoot BR guns, I'm not Not referring to BR accuracy...so maybe that's where differences arise. Again, mileage may vary, but I've seen barrel erosion 3x the rate in 22BR (or 22-250) compared with 223AI. I think you cross a threshold, and barrel life ends quickly. Stay below that, and you can expect 6,000 plus. Not sure why...just is.

 

[RAG]

RAG -

Howdy !

Not in any necessary order.....

Renowned ballistician Homer Powely has stated:
" High pressure is high temperature, and therefore; fast erosion. It is as simple as that ".

" The ballistic efficiency inside a gun is determined by the pressure. Therefore, we can say that efficiency in any gun is going to be limited by the limit of pressure which can be used ".

FWIW -
There was a rifle/ cartridge " overbore " chart that was circulating around; some years back.
In .224" calibre, the chart showed " overbore " capacity began right @ the .220 Swift capacity level. By extension, then.... a .22BR and a .22-250 would not be classically
" overbore ".

I have owned / shot a .22BR benchrest rifle, and numerous .22-250s.
Additionally, I was well acquainted w/ the loads and terminal effects for both my best friend's .220 Swift; and my wildcat " .22-35 Remington ". The latter has case capacity between that of a stock .22-250 and a .22-250AI. For use on groundhog, we shot the Hornady 55SX w/ all the above cases.

Based on my .224" calibre experiences ( use on groundhog and paper targets ), I am inclined to agree with the overbore threshold listed by the chart I mentioned above.


With regards,
357Mag

Ha. Maybe all is true. I certainly can't get into all the technical jargon...only the results I have experienced. And oh my, I burned out a couple 22-250 barrels mighty fast..like 1200 rounds, and I didn't even run super hard. Yet I can run a 223AI hard and get over 4x that in barrel life to my standard. So for my "arbitrary" calculation, da 22-250 is quite overbore. But there are probably some who shot a 22-250 and hard hard/hot for 4,000....just not me.

 

[K22]

In my experience, the issue of barrel life as related to shooting is not a mystery or nuclear physics. There is no need to postulate elaborate theories to under the few basic concepts effecting barrel life.

Overbore cartridges tend to have less barrel life compared to those that are not since they push more powder in a confined space intensifying the effect. However, overbore cartridges like the great 22 250, an excellent long-range varmint / predator cartridge, have their necessary place in the shooting sports. Most don't select the 22 250 as a target cartridge but as a hunting cartridge for long range.

The other issue having a huge effect is shot sequence. Heat is the number one cause of barrel / throat erosion. When you shoot a lot of rapid fire shot sequences and overheat the barrel, regardless of cartridge type, you going to wear out the barrel quicker.

I indirectly extended the serviceable life of several 243Win barrels by restricting my range practices to cool / cold barrel sequences. I shoot this way not to purposely extend barrel life, but because I am only interested in the cold / cool barrel shot because for me, a hunter, that is the "money" shot.

 

[VA_XTC_Shooter]

RAG -

Howdy !

Not in any necessary order.....

Renowned ballistician Homer Powely has stated:
" High pressure is high temperature, and therefore; fast erosion. It is as simple as that ".

" The ballistic efficiency inside a gun is determined by the pressure. Therefore, we can say that efficiency in any gun is going to be limited by the limit of pressure which can be used ".

FWIW -
There was a rifle/ cartridge " overbore " chart that was circulating around; some years back.
In .224" calibre, the chart showed " overbore " capacity began right @ the .220 Swift capacity level. By extension, then.... a .22BR and a .22-250 would not be classically
" overbore ".

I have owned / shot a .22BR benchrest rifle, and numerous .22-250s.
Additionally, I was well acquainted w/ the loads and terminal effects for both my best friend's .220 Swift; and my wildcat " .22-35 Remington ". The latter has case capacity between that of a stock .22-250 and a .22-250AI. For use on groundhog, we shot the Hornady 55SX w/ all the above cases.

Based on my .224" calibre experiences ( use on groundhog and paper targets ), I am inclined to agree with the overbore threshold listed by the chart I mentioned above.


With regards,
357Mag

Where did you find the Powley info? Is it a book?

 

[VA_XTC_Shooter]

Where did you find the Powley info? Is it a book?

Found it

http://www.embscomputerart.com/pdfs/powleysmanuals1.pdf

 

[RAG]

In my experience, the issue of barrel life as related to shooting is not a mystery or nuclear physics. There is no need to postulate elaborate theories to under the few basic concepts effecting barrel life.

Overbore cartridges tend to have less barrel life compared to those that are not since they push more powder in a confined space intensifying the effect. However, overbore cartridges like the great 22 250, an excellent long-range varmint / predator cartridge, have their necessary place in the shooting sports. Most don't select the 22 250 as a target cartridge but as a hunting cartridge for long range.

The other issue having a huge effect is shot sequence. Heat is the number one cause of barrel / throat erosion. When you shoot a lot of rapid fire shot sequences and overheat the barrel, regardless of cartridge type, you going to wear out the barrel quicker.

I indirectly extended the serviceable life of several 243Win barrels by restricting my range practices to cool / cold barrel sequences. I shoot this way not to purposely extend barrel life, but because I am only interested in the cold / cool barrel shot because for me, a hunter, that is the "money" shot.

I wholehearted agree. So many variables impact barrel life, obviously, it's difficult to wrap your head around. I personally believe, holding all other variables similar, I would get 3x the barrel life running a 223AI at high pressures, say 3,700fps with 53s than you could with a 22-250 running low pressures to achieve that same 3,700fps with 53s. And I can say, if I take both these cartridges and run each equally hot...so running the 22-250 close to 4,000fps, the barrel life of 223AI would be in the order of 4-5X greater; I know, because I've done it, lol.

Of course, how hot your fire, bullet bearing surface, powder choice, etc, are all contributing factors. I will say however, I've been starting to think that shooting a barrel hot has less of an impact than I used to think it did, as I've been getting very bad about shooting (on the towns) until my barrel hot and I haven't noticed a huge difference compared to when I used to be more careful about not doing this...but I've also switched powders, so don't take that to the bank, lol. But I have not had the will power to primarily cold fire a rifle...maybe someday .

*My 6BR to 6 Dasher analogy might not have been the best, I admit. I've only burned out 1 Dasher barrel...and the rate of erosion was probably 3x that of my 6BR and I quickly decided this was not the dog town cartridge for me, but I also shot a lot of heavy bullets from the Dasher compared to bullets with ultra short bearing surface (55s) at higher speeds in the 6BR. Who knows...is it the short neck, longer bearing surface of the, higher powder volume, or just bad luck that caused my Dasher to shoot out so fast.

 

[mikecr]

I researched this ~25yrs ago, using ~10yrs of info posted at every gun related forum at the time.
I've read all the books and watched it from my guns and others for the past 47yrs of my reloading.

The hardest part about research here, is a lack of standard for when a barrel's life ends.
In a sense, it doesn't end. Instead, at some point you concede that it's not useful for YOU anymore.
A 6PPC shot so much that it passes 1/2moa ability at 100yds can still be useful to someone wanting only 1moa. So 1moa dude ends up claiming 10,000 rounds barrel life for a 6PPC, while competitors are swapping them out consistently at 900.. Who do you believe? Well, who is most consistent?

The answer to your questions is not tied to barrel life, but ACCURATE barrel life.
While this is still not an exact number, it is easier to define, and it is way more consistent.
Accurate barrel life is a point passing peak potential of a barrel
For the guy who actually enjoys peak performance from a barrel, he will see it leave within a range session one day. If stubborn, he can go through a lot of efforts to pull another 100-200 good but fleeting shots. Eventually, he should learn that it isn't worth it, and hopefully he logged his accurate barrel life where it forever step changed to worse than peak. Hopefully he has another barrel in waiting for this.

RAG, if you consider this, your perspectives should change a bit.
Then it's easier to go through what affects what for you.

 

[Laurie]

and high speed bullets cause barrel wear?

The barrel wear that determines its life, or as @mikecr correctly says, its accuracy life, is wear to the throat / leade area and the first few inches of rifled barrel. Bullet to rifling friction wear is minimal by comparison in all normal situations and isn't a factor here.

Push more gas (larger case / charge) down a smaller hole and throat erosion is accelerated.

Increase the pressure and temperature of that gas, then even more so. Even 'under bore capacity' cartridges (if there is such a definition) such as 308 Win will wear a barrel out in a relatively short number of rounds if loads and pressures are run really high - which small primer brass allows in the 308. I know of many 308 FTR stainless barrels that were shot out in between 2,000 and 2,500 rounds of such loads - and we pairs-shoot in the UK, not 'string' which gets barrels a lot hotter. Yet in British and British Commonwealth prone 'Target Rifle' in the days of issue military 7.62mm ball ammunition, 5,000+ rounds was the norm, 7,000 rounds not unusual, and some stalwarts achieved 10,000 or more rounds on their practice rifles before a re-barrel was needed. To be fair, at 10,000+ rounds, the rifle would usually only just hold the 2-MOA 5-ring 'Bull' elevation-wise, so every shot had to be near wind-perfect, and the V-Bull count (X-count on the western side of the Atlantic) would be low. These barrels (at least those in the British Isles, not Canada, Australia, or New Zealand) also had the benefits of being shot in a country with low ambient temperatures, and in all cases alternate shooting 2-on-the-mound, or even 3, so barrels stayed cool.

As some previous posts have argued / discussed (and apparently disagreed over), it's as clear as mud as to how much wear a higher expansion rate cartridge design vs a smaller one run at much higher pressures compare when it comes to wear / barrel life. Combine high values of both metrics and wear is going to be bad; combine low values and it will be good - hence 6.5Grendel barrel life, and even its smaller calibre offshoot the 6mm Hornady ARC should give excellent life (but much less so if loaded very 'hot' for bolt-actions).

The other factor not so far mentioned is the nature of the wear. Anybody who has shot old late 19th century / early 20th service rifles with their modest case capacity to bore ratio cartridges and pressures of 50,000 psi or less, but often with high round counts imparted by their former military owners will have encountered erosion / extended freebore. Likewise buying well-used 1st and 2nd gen 7.62mm TR/Palma rifles. I've owned several over the years where a COAL measurement with the Hornady/Stoney Point OAL case-gauge, not only saw the bullet shank out of the case-neck, but the entire bullet and the push-rod sticking out well ahead of the tool's modified case. In such instances, the erosion was invariably smooth and the rifles still shot reasonably well albeit at considerably reduced MVs. Today's high pressure handloads allied to stainless barrels sees very rough throats and start to the rifling. The giveaway in multi-stage matches is when you see competitors cleaning / decoppering like mad between stages, the barrels rapidly lifting excessive amounts of metal off bullet jackets. Not only does precision then go down the drain but pressure goes through the roof and the shooter often retires once primers start blowing. The first such 308 barrel I wore out with a combination of the then new 185gn Juggernaut and hefty charges of Viht N550 suffered the excess coppering and need for cleaning at the ned of its life, but then stopped shooting mid match barely holding the target frame at 1,000 yards. A borescope examination not only showed the usual bad firecracking and erosion, but a chunk of land had been burned out just beyond the leade leaving what looked in the borescope like a huge crater or pothole.

 

[357Mag]

There is general confusion as to the meaning of the term 'Expansion ratio'. As I understand it, the first of your definitions is the strict (ballistician's) definition of the term. As you note, that introduces a firearm related variable that is quite unrelated to the cartridge design.

I have seen an unofficial term in use for the latter, the 'Effective Expansion Ratio' that is purely cartridge design related. For example how far does the bullet travel to double the volume of the combustion chamber. In a 45-70, that's a much shorter distance than in 243 Winchester.

The 'Over bore' [capacity] type metric is the ratio of case capacity to bore diameter. The easiest (and most commonly used) way to calculate this is fireformed case 'overflow water capacity' in grains weight as used in QuickLOAD, GRT etc divided by the bore size area in square inches. There is no formal / official definition of where the 'over bore capacity' boundary lies, partially because the availability of very slow burning powders keeps increasing. A previously over-bore cartridge design that gave little or no additional MV for the additional powder charge weight over lower case capacity models in P O Ackley's days when he considered IMR-4350 and 4831 as exceptionally slow-burning magnum powders is much less so today with Retumbo, Re33, 50BMG powders etc available to the handloader. Nevertheless the 7mm Remington Magnum with a case capacity to bore area ratio of 1300 was long regarded as the borderline, and is as good a boundary as any even today. (Today's 7WSM which many users define as a finicky number to load for precision use is also 1300 and can wear barrels out very quickly if loaded up to its maximum. The 300WSM using the same basic case under a bigger bore at c. 1,080 is a much better mannered beast, regularly noted for being easy to tune with wide accuracy nodes. I shoot the 300 SAUM which with c.987 is a real sweetie, very easy to tune indeed. 308 Win at the 750 mark is hugely powder and load tolerant and gives great barrel life if loads / pressures / firing rates aren't abused.)

This ratio is one of the major determinants of the optimal powder burning rate required as well as barrel life. The higher the value, the slower-burning the powder needed. Bullet weight is also a major factor too, as nearly everyone knows. More open to argument, heavier bullets also wear barrels out faster too in that more mass = greater inertia = the bullet spends longer time in the throat/leade area under maximum pressure, and ...............



extreme heat for longer in the bit of the barrel that determines its life.

Another new factor which has the same effect is Swiss chemical company Nitrochemie Wimmis AG with its patented 'EI' powders as seen in a few Alliant Reloder grades and some of the Reload Swiss grades we have in Europe. 'EI' infuses the deterrent chemicals into the kernels under the surfaces which therefore work for more of the burn than traditional simple surface coatings. This extends the controlled burn period in time / bullet travel and allows the peak of the pressure curve to be longer/flatter. As with heavier bullets it means peak pressure lasts for a longer time, but also now bullet travel too. As this increases the area under the pressure curve it improves MVs - drastically in some cases such as RS60/Re17 in the 6XC and 284 Win with match loads getting boosts of a claimed 200 fps in some combinations. Extended peak pressure = extended period of peak heat energy = increased wear as people who maxed loads / pressures out with this powder soon discovered especially in BR and US F-Class with string-firing and rapid heat build-ups.

Years back somebody on this forum posted jpegs of an article written by a Finnish ordnance officer of ES and group size growth vs round count in the NRA US Rifleman magazine using the Finnish 7.62X53R service rifle with a standard but selected barrel. 7.62X53/54R has a bore capacity ratio of c. 850, or 100 higher than 308 Win, but its nominal maximum peak chamber pressure is 390 MPa (56,565 psi) under modern CIP methods and standards. I suspect the service cartridges used in these Finnish Army tests were somewhat, possibly considerably, lower pressure. ES values, likewise groups, only started to increase slightly after the barrel was at 11,000 rounds from new, likewise MVs started slowly dropping. The tests were discontinued at over 13,000 rounds with the rifle still shooting well, but the testers worn out. I don't know what effect the qualities / hardness of Finnish chrome-moly barrel steel had on the outcomes compared to the relatively soft 'stainless' we use in match barrels, but the relatively low case capacity to bore ratio and chamber pressure values must playlarge part in providing this level of barrel life. 65Grendel forum members say their AR-15 6.5 Grendel (capacity value 665 based on Lapua in my rifle's chamber); SAAMI 52,000 psi max pressure) barrels (presumably stainless) last 9-11,000 rounds - and I don't suppose most such shooters shoot strings of five rounds slowly, then let barrels cool before resuming! Hearsay I know, but I wouldn't disbelieve it. Scandi 6.5X55 shooters with their Sauer STR200s using modern chrome-moly steel barrels and mainly factory ammo which I presume sticks to the old Swedish Mauser max pressure around 50,000 psi (less?) report excellent barrel life, far better than many people I know using hot 6.5 Creedmoor handloads with heavier bullets in stainless barrels (1056 capacity ratio vs 975 for 6.5X55 and 6.5 Creedmoor).

So whichever way you play it, military scientific testing or hearsay, any of the individual metrics of high chamber pressures, high bore capacity ratio values, high rate of fire reduce barrel life all other things staying the same, but in combination will severely reduce it.

Laurie -

Good stuff ! Thanx for

Found it

http://www.embscomputerart.com/pdfs/powleysmanuals1.pdf

VA XTC -

Howdy !

Glad you found the references.


Regards,
357Mag

 

[RAG]

The barrel wear that determines its life, or as @mikecr correctly says, its accuracy life, is wear to the throat / leade area and the first few inches of rifled barrel. Bullet to rifling friction wear is minimal by comparison in all normal situations and isn't a factor here.

Push more gas (larger case / charge) down a smaller hole and throat erosion is accelerated.

Increase the pressure and temperature of that gas, then even more so. Even 'under bore capacity' cartridges (if there is such a definition) such as 308 Win will wear a barrel out in a relatively short number of rounds if loads and pressures are run really high - which small primer brass allows in the 308. I know of many 308 FTR stainless barrels that were shot out in between 2,000 and 2,500 rounds of such loads - and we pairs-shoot in the UK, not 'string' which gets barrels a lot hotter. Yet in British and British Commonwealth prone 'Target Rifle' in the days of issue military 7.62mm ball ammunition, 5,000+ rounds was the norm, 7,000 rounds not unusual, and some stalwarts achieved 10,000 or more rounds on their practice rifles before a re-barrel was needed. To be fair, at 10,000+ rounds, the rifle would usually only just hold the 2-MOA 5-ring 'Bull' elevation-wise, so every shot had to be near wind-perfect, and the V-Bull count (X-count on the western side of the Atlantic) would be low. These barrels (at least those in the British Isles, not Canada, Australia, or New Zealand) also had the benefits of being shot in a country with low ambient temperatures, and in all cases alternate shooting 2-on-the-mound, or even 3, so barrels stayed cool.

Yes, I feel like the amount of gash pushed, relative to the bore diameter, is by far the biggest factor. At least it has been for me. I "felt" like, the pressure was a much smaller factor, as I have ran a few 22" 6BR barrels generally in the 3900 fps with 55s (which is pretty hot./fast for a short barrel) and I typically have my groups open up from "around" .25" to about .4", after about 6,000 rounds, which is usually when I retire them. So this is "Varmint" accuracy for "Me". I have one such barrel at 5,500 rounds that still likes to give me some quarter inch 5-shot groups. BUT, you mention temperature of the gas...I usually run ball powders that I believe run lower temps and help preserve barrel life. POINT BEING...I've had a .243 Win before too, and I don't think it would give me 1/3rd that, even when ran at lower pressures to generate the same velocity; hence I feel like the case capacity is the predominant factor.

As for those 308s being shot out after 2K-2,500 due to being run at really high pressures, that is interesting in fact. I dunno. Maybe it's because we're talking "competition accuracy", so the groups opening up by just .1" is shot out.

As some previous posts have argued / discussed (and apparently disagreed over), it's as clear as mud as to how much wear a higher expansion rate cartridge design vs a smaller one run at much higher pressures compare when it comes to wear / barrel life. Combine high values of both metrics and wear is going to be bad; combine low values and it will be good - hence 6.5Grendel barrel life, and even its smaller calibre offshoot the 6mm Hornady ARC should give excellent life (but much less so if loaded very 'hot' for bolt-actions).

The other factor not so far mentioned is the nature of the wear. Anybody who has shot old late 19th century / early 20th service rifles with their modest case capacity to bore ratio cartridges and pressures of 50,000 psi or less, but often with high round counts imparted by their former military owners will have encountered erosion / extended freebore. Likewise buying well-used 1st and 2nd gen 7.62mm TR/Palma rifles. I've owned several over the years where a COAL measurement with the Hornady/Stoney Point OAL case-gauge, not only saw the bullet shank out of the case-neck, but the entire bullet and the push-rod sticking out well ahead of the tool's modified case. In such instances, the erosion was invariably smooth and the rifles still shot reasonably well albeit at considerably reduced MVs. Today's high pressure handloads allied to stainless barrels sees very rough throats and start to the rifling. The giveaway in multi-stage matches is when you see competitors cleaning / decoppering like mad between stages, the barrels rapidly lifting excessive amounts of metal off bullet jackets. Not only does precision then go down the drain but pressure goes through the roof and the shooter often retires once primers start blowing. The first such 308 barrel I wore out with a combination of the then new 185gn Juggernaut and hefty charges of Viht N550 suffered the excess coppering and need for cleaning at the ned of its life, but then stopped shooting mid match barely holding the target frame at 1,000 yards. A borescope examination not only showed the usual bad firecracking and erosion, but a chunk of land had been burned out just beyond the leade leaving what looked in the borescope like a huge crater or pothole.