BARREL BURNING THEORY

[camac]

Guys,

I have been working on a barrel burning theory based on waste energy (heat) and it seems to be coming out almost spot on - but as always needs some work.

The idea: heat is created from waste energy from the combustion of the powder and primer and barrel wear is directly related to the amount of heat produced and dissipated (Other things have an abrasive and compressive effect but this is accelerated with heat input) I have taken this a lot further but thought I would post this bit by bit and get the forum groups valuable insight whilst developing it. Eventually I would like to have the actual energy input for each powder and primer etc plus other inputs.

Firstly I would like to concentrate on the idea and logic of the waste energy concept and give a few speadsheet examples of same barrel length, same bore diameter, same shot timing etc before moving on

Before anyone goes off on a tangent talking about how to save barrel life or stating that "I got 10,000 rounds out of my 300RUM". Please understand this. This spreadsheet is based on a "standard" and that standard is shooting a 30BR under match conditions. It is assumed all rifles are shot in the same way.

Ignoring primers for a while ( I will get to them later but they contribute a small amount and generally we increase the power of the primer in proportion to the powder charge ie BR primers for small charges, magnum for large charges). I used the 30BR as a reference point and a simple squared formula and sticking with 30 cal at the moment.

Once again I am sure someone else out there has done something similar so any references or leads would be handy. Cheers.
Cam

ps have since read mikecrs formula spreadsheet and "fixed" my powder potential energy figures.

 

[TonyR]

Cam. Take a look at this. http://bulletin.accurateshooter.com/2008/10/spreadsheet-formula-calculates-useful-barrel-life/
There is a thread way back were this was discussed and naturally the best example of how it didn't work was the 30BR. Actually seems to work reasonably well with the 6BR and the various 284 variations. Good luck Another interesting thread.

 

[camac]

Thanks Tony, Didn't see that and definately feel a little foolish now so I will go and look at Mikes work and see if I can get my head around it. - I read some other posts and thought they were so far off I had better take a look. but didn't do a big lot of research as to what had been written and things came together pretty quickly (assuming loading to same pressure, and non-moly bullets).
Needless to say his theory looks a lot better than the ones I was looking at.

 

[TonyR]

I found the old post:
http://www.accurateshooter.com/forum/index.php/topic,3218460.msg32128228.html#msg32128228

 

[Robert]

Camac,
I modestly think that heat is mostly dissipated into the barrel material with limited wear influence on its own.

There is certainly other factors of more importance (in target or sporting shootibg usual rates of fire), and it is to consider also, I believe:

-The thermo-chemical effects of the combustion, for instance combination of heat, carbon and nitrogen. As far as I know, this results in micro-nitridation, no?

-Mechanical effect, for instance:
a) The sandblasting effect of propulsed powder/primers solid hard residues hammering the forcing cone.
b) Friction, depending also mostly of calibre and barrel lenght. The artillery notion of qualifying the tube lenght by its number of calibres coud find its importance here? A 28 inches (roughly 30’ total length ess the chanber) .30 barrel is 90,9 calibres, while a .223 is 125,5 calibres. Frictopn occurs and consum enrgy all the way long. This is also influenced by bullet/bore diameters difference and height of bearing surface.

-I understand you have temporarily imited your theory development to the .30, but I am already waiting to see this extended to other calibres….

-You took as a reference a .30BR with a 110 grains bullet? The only use I can find for a 110 grains .30 bullet is for the .30M1 carbine…or as a plug for cheaply fire-forming other case capacity calilbres!!!.

-It would be interesting to know some equivalences with the ADI powders you mention. Are they single or double-based?

Let us know the rest of your observations and conclusions as soon as they come out. As Tom said, an interesting discussion.

R.G.C

 

[RonAKA]

Your method seems to give quite similar results to a formula found at this link:

http://yarchive.net/gun/barrel/barrel_life.html

Using it I get the following for the calibers you have in your spreadsheet (in the same order).

5990
3419
3005
1589
958

For Robert, ADI makes most of Hodgdon's extruded powder. I believe 2208, 2209, and 2213 are identical to Varget, H4350, & H4831 respectively.

http://www.adi-powders.com.au/handloaders-guide/equivalents.asp

The formulas could probably be refined further by putting in a factor for the high temperature heat resistance of the barrel material. Another slight refinement may be compensating for barrel length or better still barrel time. I suspect, the longer the barrel the longer the duration of the heat, and more burning takes place. Heavy bullets may increase barrel time too.

 

[mikecr]

I found the old post:
http://www.accurateshooter.com/forum/index.php/topic,3218460.msg32128228.html#msg32128228

This post led to a faulty spreadsheet. Not sure how I screwed the pooch on that one.
I'll attach a better copy here

Camac, I wonder how to get a good start using 30br as your 'reference' cartridge. Is there barrel life validation data for it?
I'd be surprised if anyone has burned one out yet..
And 30cals in general are very subjective here. I've heard claims of 20Krnds+ for 308s. Actually, I've heard claims of every stretch with 308s, and wonder sometimes about those who shoot it..

IMO, you'll get further examining barrel life where the cartridge used consistently causes them to reach a clear end.
The attached spreadsheet demonstrates a common 6PPC load of 28gr N133 behind a 68gr bullet. QL arrives at ~70Kpsi for this assuming the bullets are jammed. Now with a short shot rate that these poor barrels are subjected to, accurate life drops off dramatically(to ~1000-1500).
Yet a 6br fired normally, with sane loads(albeit more hotter powder), could be expected to last twice as long.

What you're piecing together can easily fail tests.

 

[TonyR]

Thanks, Mike. I had hoped you would find this post and join in. Your spreadsheet has been very helpful.

 

[camac]

Thanks for the input guys. I actually have a few extra columns in my spreadsheet here (including barrel diameter, shot timing and pressure) but only wanted to post one at a time in order of what I found to be the strongest correlation and get as much discussion as possible on each topic. Now I have seen MikeCR's page I feel it may be a bit of replication and do not want to take away from his nice work. - good to drag his stuff back to the fore anyway.
We may just limit this discussion to the waste heat energy concept and leave out the pressure formulas etc as Mike has already covered it.

Robert, I do agree I actually think what is happening isn't the heat alone but the steels resistance to all the abrasive and compression forces gets much weaker under heat - in a power relationship. As far as a reference I think the point of the analysis is you could use any "reference" point - shot in your style, in your barrel length, in your barrel steeel and then use waste heat energy to give you a fairly accurate estimation if you changed calibers but kept the same shooting style and pressures etc. Indeed there are a lot of variables that can affect the barrel life but if you compare a horse with a horse it is a solid relationship. If you want to change barrel diameter or pressure then we can introduce a reltionship there too but once again, Mike has done that.

MikeCR - thanks for chiming in - I hoped you would too as you probably have done more work on this and understand it better than any of us and I value your opinion. Indeed the 30BR is probably not a good reference point as it can vary so much from being shot out in 6000 to going well over 10,000 (or more) but in reality what I was going to show is in my limited experience you could put any reference you like in there if you know you shot out a barrel in "x" rounds. At our range most of the 308's get replaced between 3-5000 rounds. I propose that if you shoot the other calibers at the same pressure, in the same style of fire then the simple relationship will hold fairly closely. I have actually worked through a lot of other variables as well but always came back to this being the strongest relationship (followed closely by shot timing). I will plug in a few figures if you like on 6mm using a reference point of 6BR and 2500 rounds. I agree a high pressure load will reduce it somewhat and even more so the firing rate (after all the waste heat energy can't get away) running a 6BR vs 6*47 vs 243 at same pressure and same firing rate seems to be a pretty strong relationship with waste energy alone. We can introduce columns for these but fundamentally I believe the damage is being done by the heat (and then friction, compression and other abrasive forces). The relationship is strongest with heat generation and firing rate (or the ability of the heat to escape) but other factors will have some influence too.

 

[camac]

MIKECR, interestingly I tried changing the shot rate of your above 6PPC load to 60 sec on your spreadheet (like our F class matches) and came out with 3900 rounds. I plugged the figures into my spreadsheet and got 3617!

Considering I am not sure yet whether the x squared fit equation is the best for this model (I actually think an exponential function with a fixed reference point will fit better) I am still thinking it will get close. Waste heat energy goes up with pressure as you increase the load.
Whilst I essentially think your formula is excellent, I still think the "reason" for it has more to do with waste energy and heat than pressure although they both are tightly related and hence you could plot a fit off each.

The shot rate (which is basically another factor of heat effect - heat dissipation) and barrel diameter would be next on the hit list in importance. I have added columns for both these and was surprised at the power of the equation that was needed for both! but once again I would like to get the "waste energy" concept discussed through before adding those in this thread.

 

[RonAKA]

Have you accounted for the energy that goes into the shoulder of the shooter?

 

[camac]

I did in one of my spreadheets: First of all to summarise: As we all know one of the fundamental laws of physics "Energy cannot be created nor destroyed" just transformed. So the potential energy of combustion of primer + powder is turned into kinetic energy of bullet + kinetic energy of rifle and shooter going backwards + kinetic energy of propellant gases escaping out the muzzle + noise energy + heat energy (any others??). As the recoil energy of rifle is so low (generally 4-30 ft lbs) compared to thousands ft lbs for muzzle energy it can be ignored or added if you prefer. Probably more significant are the other two. Kinetic energy of escaping gases and particles and noise energy. However, all of these seem to go up fairly proportionally with waste heat energy so for the accuracy of these estimations can be lumped together as "waste" and waste is proportional to heat created.

 

[TonyR]

cam. It seems that you need to address the temperature on the surface of the bore and how that effects the resistence of the steel to erosion at different temperatures. Clearly there has to be a heat balance, but it also seems clear that the bore has a fixed area and a constant heat transfer coefficient so it would appear that the temperature differential from the inside of the barrel to the outside is going to increase as the rate of fire or any other factor increases the energy input to the inside of bore. That may or may not change at the same rate that the surface temperature of the barrel changes. I wonder if there is a reliable way to estimate the surface temperature of the inside of the bore under different conditions of load and/or rate of fire??

 

[mikecr]

camac, it's nice to see an endeavor towards the truth.
My spreadsheet is based on nothing more than an expanded rule of thumb(like an enhanced Greenhill). It therefore does not represent any truths.
It is useful however, in that it's been tweaked and validated over many years of forum discussions related to barrel life. Checked against pretty much everything, common to extreme.

A gunwriter named Ken Howell put out a book(I don't remember which), in which he explored barrel life deeply. Ken was all about cartridge design. I believe he concluded that Powder amount was dominant, followed by shot rate, then pressure. But surely powder amount is influenced by it's heat generated/dissipated -within a given area. And so powder burn RATE might also affect barrel life. And so Pressure, which affects powder burn rate would come into play. He also theorized that pressure of itself increases temperature specifically in the throat(at pressure peak), regardless of powder burning attributes.
Later, Henry Childs(a mechanical engineer) explored barrel life more from your approach. He strongly believed that powder temperature(caloric energy I think) was more significant than commonly thought. He also pointed out that moly coating extended barrel life because of the latent heat it took from combustion, and not from frictional reductions.
Just stuff to consider.

I have nothing but reasoning. And although barrels are often burn out with leade receding, I think the ultimate killer of barrels is carbon impingement(choking grooves). This, because a barrel can only be setback so many times before it finally dies. Even regular use of JBs, combined with setbacks, will not keep a barrel going forever..

Anyway, you never know, you might discover something easier about it.

 

[camac]

Thanks again guys.

Tony, yes you are right the surface temperature of the bore or more particularly the raised rifling seems to be the key. I truly believe all those abrasive, corrosive and compressive forces go up with temperature, As you get close to melting point it becomes exponential. In Mike's and other study's and this theory the main factors appear to be how much heat is created and then how it is dissipated before the next shot. Bore diameter is also major but seems to come into play from a heat concentration point of view. Is there a theme here... Heat, heat, heat. I like your thinking on trying to measure or estimate bore temperature and will have a think about that. Stainless is a terrible conductor of heat and "standard barrels should be much better at dissipating it. Mike's shot timing logic seems to hold a good rule of thumb though.

Mike, Thanks for the reference leads, I might try and chase them up when I get time. I have been struggling to come up with a reason as to why pressure matters and what you say makes sense so I will have a few more coffees and try and get this old brain wrapped around it. The interesting one is carbon build up and I agree totally here too. Why does it hinder barrel life so much? I am not really sure. Perhaps it acts as an insulator or heat sink, perhaps it is a pressure thing?? Perhaps Moly reduces carbon buildup??

 

[camac]

Mike, I tossed around the "pressure" concept last night and can't for the life of me work out why this would make a difference. I may very well be wrong and am very underequipped with references (and probably knowledge and quickload) but still can't get my head around it. I couldn't work out any scenarios where I would be increasing pressure without increasing powder charge, or powder burn rate or energy (and hence heat energy), or reduced barrel diameter so gave up. I do think perhaps pressure is a good indicator of some other things at play (perhaps burn rate so indeed hotter at start of barrel? but have to dig into this further. I would imagine you have a lot of real life data points on this and wonder if you can give me some examples. I am happy to take whatever you can give me and run them through if you don't mind?? cheers.

 

[TonyR]

cam, I think the issue with pressure is that the waste heat increases as exponentially as the pressure nears the maximum. I have noticed increased ES when my ladder groups approach the QL maximum and I think the proportion of total energy available that becomes waste heat increases exponentially after a certain point and the whole combustion process become less stable.

 

[camac]

cam, I think the issue with pressure is that the waste heat increases as exponentially as the pressure nears the maximum. I have noticed increased ES when my ladder groups approach the QL maximum and I think the proportion of total energy available that becomes waste heat increases exponentially after a certain point and the whole combustion process become less stable.

Tony - agreed - But I think I am asking a bit of a "chicken or egg" question.

A good comparison here would be a 6PPC vs 6BR again as Mike pointed out but this time put the same powder charge and projectile in them. For instance 107Gr SMK, BR2 Primer 28 grains of VARGET (ADI 2208) the 6PPC is more efficient and will push it out at 2650 from a 24 inch barrel but it is near max load. The 6BR only at 2500fps or so. The 6PPC will run at higher pressure (guessing 65,000psi) and the 6BR lower but the wasted energy is higher from the 6BR.
So - which would win with regards to barrel life? According to Mikes formula and the pressure theory the 6BR would by about 500 rounds, but I think (with no proof mind you) the extra waste energy from the BR would be converted to heat and cause it to have a shorter life if shot at the same firing rate. I think at 1 minute shooting interval they would both go past 3000 rounds and be too tight to test anyway. But interesting to contemplate nonetheless.

 

[TonyR]

Hard to tell without shooting a few thousand rounds I guess but I am curious about the PPC. As far as I know, the case is rated at 58000 psi. I am not sure where people actually shoot it, but I get great accuracy when the nodes coincide with loads that QL estimates to be in that vicinity.

 

[camac]

I don't have QL but is it possible for someone to run the above two loads up please??. It would be interesting to see the pressure difference in same powder charge between the two cases