How Hot to "open up"

[falconpilot]

Got a question..How hot would a barrel have to get before it "opens up" at little and starts throwing shots? I'm talking in theory??? The anwser is pretty easy...when it starts throwing shots!! Is this line of thought founded and reasonable to assume such? I've fired quite a few perfect score in F-Class with my Dasher, and it has always had perfect vertical, on the hottest of days through long strings of fire..ie.20+....I shot very fast....

My 284 Shehane, and my current 284 both shoot really well through about 15 shots or so, and then they start opening up, throwing a shot....This has me wondering if the barrel on these larger caliber guns are just plain getting to hot due to their larger case/powder cap. ,especially now, in the summer, and throwing shots at the end of strings...It can't be the load..other wise, I wouldn't be able to shoot 15 clean, with many x's, time and again, only the have the string go to hell after the 15th shot....I'm average 196/197's, but I'm always dropping a couple points out the vertical, and always toward the end of the string...any thoughts??

 

[spclark]

I'm inclined to agree with your reasoning on this; the last four or five shots are experiencing chamber and barrel conditions quite different than the first ten or fifteen.

Just this past week I recall seeing something somewhere about how each 1° rise in barrel temps causes pressures to increase upwards of 250-300 psi. If you're already near the ragged edge at the start, ten or fifteen rounds later you could be seeing 3000-4500 higher psi.

You shooting plain or coated bullets?

Could be propellant too - not all are suitable for extended strings of fire. You want to fill in some details on your load particulars?

 

[Steve Blair]

You might try replicating your match firing pace over a chronograph and see if velocity changes as the barrel heats. When I am developing loads, I use a digital thermometer to monitor barrel temperature. When it reaches 100° F just forward of the chamber, I let it cool.

http://www.amazon.com/KINTREX-IRT0401-Waterproof-Infrared-Thermometer/dp/B002MYMSOI/ref=sr_1_1?ie=UTF8&s=hi&qid=1281824897&sr=8-1

The powder might be the problem. I shoot 7828 SSC in .284 with 175 and 180 Bergers and have not observed this effect.

 

[fdshuster]

I can't comment on any of the "larger" calibers, but I know for a fact that given the same rate of fire/ number of rounds fired over the same time period, my 22-250 Varmint contour will heat up a lot faster than my 222 also with a stainless Varmint contour. Same bore diameter, one with 35.5 grs. (H4895) vs. 20.5 grs. (H4198).

 

[GermanS1]

I suspect your barrel heating up is part of the problem (maybe all of it). However, Steve's estimate is 250 psi per degree seems very high, a barrel can easily increase over 100 degrees during a match. Mine is at 100 degrees or more before I ever fire the first shot, and there is no way to hold it after a string, you would lose flesh. I don't get vertical at the end, but you would see the vertical easier in F-Class perhaps.

Hatcher's Notebook (1962 edition) at page 399 shows that Frankford Arsenal's testing of the .30-06 (with IMR 4895 no doubt) increased MV 1.75 fps per degree between 70 degrees and 140 degrees. In my .30-06, for a 5700 psi increase (from 52.0 gr. of 4350 to 54.0 grains) I get a 123 fps increase, so that's about 46.3 psi/fps. Accordingly, if each degree increases MV by 1.75 fps, it is increasing pressure by about 81 psi (46.3 psi/fps) x (1.75 fps/degree) = 81 psi/degree.

Some of today's Hodgdon powders might show a reduced change with temperature, but don't assume it's eliminated. Even a small reduction is significant enough to merit their advertising claims.

Now, for the full effect, the cartridge would have to sit in the chamber long enough for the heat to soak through the brass into the powder and primer and with the rate of fire in F-Class, that's unlikely to happen. However, there is some effect.

A good procedure is to unload immediately after the shot, place the next round in the rifle but don't move the bolt forward until the target starts to rise from the pits. That seems to help me keep vertical in check.

 

[falconpilot]

Load data..

284 Win. Using necked up Lapua no-trun neck brass. Powder 4831sc 54.8grs, 180 Berger Hybrid seated .015 inot lands. Bartlein 1:9" 32" tube...
Dual port panda..I always jsut lay the next round in the tray..I never bolt it in and let it set to cook off..

The vertical is not in jsut one direction...First shot went to 2 oclock in the 9 ,and the next went 7 low 9, followed by the next back into the x ring..held center all shots..

Jim

 

[pdog2225]

I would like to apply a little of "Spocks" logic to this subject.
I've been told that metal expands as it takes on heat. If this is true then the barrel must expand both outside and inside also. The bore must be hotter than the outside of the barrel because that is where the heat is generated. This means that the bore is getting smaller in diameter as the heat increases. This would explain some of the increase in velocity/pressure as the tube heats up. Then when the barrel gets hot enough the bullet(s) upon shooting will be swaged into a smaller dia. bore, causing it to be distorted to some degree.

Regards pdog2225

 

[spclark]

However, Steve's estimate is 250 psi per degree seems very high...

I mis-remembered the pressure value apparently; it's more on the order of 175-200 PSI / °F.

I found the original info, linked from a bulletin entry dated August 2nd:

http://www.shootingsoftware.com/ftp/Pressure%20Factors.pdf

The bulletin excerpt implies barrel temp is more important than powder temp. The author, in the complete article, states he suspects that barrel temp affects primer temp which results in the velocity increase per degree, rather than any expansion or tightening of the bore boosting pressures.

German, you've done exhaustive testing of various primers. Have you ever done anything to measure the impact of increased temps on primers?

Pdog2225 I've always thought much the same thing. A barrel doesn't expand from the center of the bore, it expands from the center of material which ought to cause a warming barrel to grow both inside the bore and outside where it's relatively cooler. My position has caused some heated arguments though & I have no practical way to prove my theory.

 

[Laurie]

I've always understood that hot barrel = moving POI is mostly down to bedding / metallurgical stress factors in the barrel steel + above barrel heat haze affecting point of aim. Hammer forged barrels have lots more internal stresses than cut rifled equivalents all other things / destressing procuedures apart, so their being notorious at one time for sometimes large shifts in POI seen after as few as half a dozen or so rounds. The speed of this kicking in and severity of the results obviously depends on the powder charge weight v bore dia ratio to a large extent.

The effect on propellant temperature is obviously heavily influenced by how long the cartridge is in the chamber before taking the shot. Having ammunition sit in the open in full summer sun might have a greater effect here over a long string of shots than chamber induced heating if shots are taken quickly.

Above barrel heat haze can have a much larger effect than many people realise even when it is mild and therefore barely or not at all noticed by the shooter. This can apply in winter with only slightly warmed air above the barrel seeing a greater temperature differential from very cold ambient air than applies in warmer conditions. The UKBRA has a programme of monthly 600yd winter BR comps with sighting-in and all four 5-round groups shot in one session (to maximise use of the available short daylight hours in our Northern latitude) with a break between the two pairs of groups to allow barrels to cool. It's surprising how often the first group is a shooter's best of the day, and the second pair only rarely match the first (assuming weather remains constant throughout the match of course). An experienced long-range BR shooter reckons that many people run their scopes at too high a power in such conditions. There is maybe something, maybe not to this hypothesis. When one is talking small fractions of MOA in group sizes, it might have more effect than more 'apparent' causes such as chamber heating.

Laurie,
York, England

 

[Steve Blair]

I would like to apply a little of "Spocks" logic to this subject.
I've been told that metal expands as it takes on heat. If this is true then the barrel must expand both outside and inside also. The bore must be hotter than the outside of the barrel because that is where the heat is generated. This means that the bore is getting smaller in diameter as the heat increases. This would explain some of the increase in velocity/pressure as the tube heats up. Then when the barrel gets hot enough the bullet(s) upon shooting will be swaged into a smaller dia. bore, causing it to be distorted to some degree.

Regards pdog2225

My recollection was different regarding thick-wall tubing radial expansion; so I checked with one of our senior Mechanical Engineers. When the barrel heats, both the OD and ID expand. If you think of molecules moving farther apart, it would be necessary for both to increase. That said, the amount of ID expansion in our temperature range is very small, but not zero.

 

[HPMike800]

Looks like Sleepy is correct, from what little I can find. If anything the bore loosens with heat. I took Germans 1.72fps/degf to be barrel temperature not ambient temp.



if you lighten your load a bit and I'll bet the problem goes away.

 

[Steve Blair]

Yes the bore tightens with heat, Frankfords velocity/temp data confirms this. So, your shooting a diffrent barrel at the end of a long string then at the begining.

Mike,
Do you have a citation on the Frankford study? I do not think that barrels behave differently than other thick-wall tubes.

 

[GermanS1]

Steve, I haven't done testing of temperature effects because I don't have any instrumentation for that. Hobby efforts are always limited!

 

[Steve Blair]

German,
The question was directed to Mike's comment. Sorry for the ambiguity. I only questioned the conclusion that increased velocity meant bore diameter reduction.

 

[Travelor]

Remember, it is not barrel temperature that causes increased velocity, but ammunition temperature.

When I purchased a 50 BMG and started reading about loading the 50 BMG and the ballistics for it, the literature used ballistic information normally used for artillery. Makes perfect sense when you think of it as they seem to have long range ballistics figured out pretty well over the years and with the research behind it.

My thought is that the barrel is not totally stress free and as it heats up and cools down it starts moving a bit.

George

 

[Kenny474]

I have to think that as a barrel gets hot it has to expand, and as it expands the bore must get a few tenths tighter. Even just a few tenths could probably make a slight difference in velocity, just as carbon fouling will increase it as well. If it increases enough to push you above your node, couldn't that induce fliers as well?

Just a thought, as I have watched steel expand a few tenths just warming up in the inspection room at work.

I know ammo temp will play a part as well, but how many shooters really leave it in the chamber long enough to heat soak? The few matches I have watched, once the guys start shooting, it's usually only a matter of 30 seconds until they have all their shots off. Not near long enough to heat soak.

 

[BoydAllen]

What sort of bedding do you have? What material was used to do the bedding? Are you bothered by barrel mirage as it heats up? Perhaps coating the bullets, and reworking the load would cut down on heat.

 

[pdog2225]

This is a great thought provoking thread. So to add further thought. If the chamber and the barrel area forward of the chamber have reached the "Open Up" temp and the heat in this area has decreased the bore diameter as the bullet passes through this zone it is now some smaller in diameter. Lets say that at the mid point of the barrel to the muzzle that the temp is below the critical point (cooler barrel). So the bullet now passes into a looser diameter bore than it started out. Wouldn't this cause the groups to open up? Would there be gas blow by?After the barrel cools down it should shoot good again. Thanks

 

[Steve Blair]

There are several forces at work. Propellant gas pressure increases bore diameter behind the bullet, which decreases as pressure decreases. The bullet is "upset" to fill the bore, if it does not already, by propellant pressure. Despite "intuition", heating thick wall tube causes both the I.D. and O.D. to expand. The heating is not uniform, however. It follows a gradient across the barrel thickness according to the thermal conductivity of the barrel material. That conductivity differs between stainless (i.e.: 416R), chrome-moly (i.e.: 4140) or chrome-moly-vanadium (i.e.: 4340) steels. The bore diameter will generally decrease to original dimensions as the barrel cools.

Again, the bore diameter increase caused by heating, in the range of interest to target shooters, is very small. The temporary increase caused by pressure is more significant. In any case, increasing bore and groove diameter is much more detrimental to accuracy than decreasing it.

 

[Kenny474]

The increase of bore diameter from heat does make more sense than decrease now that I think about it a bit. If the steel expands it will only have out to go due to the inside of the barrel basically being a continuous arc.

Hitting the inside of the barrel with 50,000psi is sure to cause some expansion, I just wonder how much. And will a hot barrel expand any more than a cool barrel from this pressure? I know it takes a lot to significantly soften steel with heat, but 50,000psi is a lot of force at work. I wonder if heat will play into the amount of expansion as well.