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As the rifle is fired and the barrel heats up, will velocities increase and/or pressure?

boss - Assuming that your charge weights and internal ballistics match from round to round, and that you are starting with a squeaky clean bore - you will generally see an increase in velocity through the string. This has little to do with heat, and a lot to do with fouling. As the bullet obturates (conforms) to the bore, jacket material is deposited in the clean barrel. This fouling acts a little like a lubricant in the bore. The next bullet down the barrel rides the fouling which decreases friction. As you shoot a string, you can see an increase in velocity.

Most competition folks shoot a fouled bore for this reason. Velocity normalizes in a fouled bore. Also, many hunters have learned about cold shot accuracy in a clean bore and how their gun shoots the first cold, clean shot.

What is interesting about internal ballistics is if you remove enough bore friction, chamber pressures drop, burn rates drop, and velocity drops. This is why molybdenum disulfide (MoS2, or moly) coated bullets lower velocity. Moly is such a good lubricant, that bullets travel freely down the bore thus reducing chamber pressure and velocity. To correct this loss, you increase powder charge weights.

Thanks. I thought it was going to be the heat. I have noticed an increase after the first shots and I always go to the range with clean bores. For whatever reason, it seems to be more with a couple of 280's as opposed to other calibers I own. Don't know what caused that anomaly. I have always hunted with a fouled bore and some do that for POI reasons, but it appears that this one more reason to do so; to compute your trajectory with velocities from a fouled bore, that being only pertinent with a low SD load if one is in to minutia which is the case with some of us rifle nuts.

Boss, to take it one step further, your pressure will increase the longer your unfired case stays in a hot chamer. I have played with this phenominon extensively and found large pressure increases, especially in .22-250, .25-284 class cartridges. The shells were kept at a constant temperature and the barrel was heated up as shots were fired through the chronograph. When the barrel and chamber were too hot to touch I left a loaded shell in the chamber for 30 seconds to a couple of minutes. Velocities steadily increased the longer I left the unfired shells in the hot chamber. In both calibers I had velocity increases of over 175 fps. at their peak. When the barrel was allowed to cool back down (still fouled and uncleaned) the velocities dropped back down to normal and pressures were okay. I repeated this several times with the same results. I didn't want to destroy a barrel, and pressures were getting pretty high, the longer the cases were in the hot chamber, so I stopped. These experiments were with H414, H4831, RL22 and AA2460. All showed significant pressure and velocity increases when left in a hot chamber. H414 and AA2460 showed the highest pressure and velocity increases. I don't know if it is because they are a ball powder or if it is because of the chemical makeup. The new Hodgdon Extreme powders are supposed to be less temperature sensitive, but I have not used them yet. Flinch

Thanks. I suspected that and never put a cartridge in the chamber while chronying until some time has elapsed for cooling and then I go ahead chamber and fire within a few seconds. I have read, too, that H414 spikes. Mule Deer recommends Ramshot Big Game as a substitute for H414 I believe.

Today at the range I got these results, after one fouling shot, three groups of three,the velocity remained at 2800+,but the pressure ring measurements dropped.The groups were one grain increases.This was in a 264.The barrel was cold for the start of each group and barely warmed what with the time for careful mike work..The powder and bullets were N-560 and Hornady SP 140 .I have no idea what this means.

ZeroDrift
Help me to understand your earlier post. In the first paragraph you stated: "As the bullet obturates (conforms) to the bore, jacket material is deposited in the clean barrel. This fouling acts a little like a lubricant in the bore. The next bullet down the barrel rides the fouling which decreases friction. As you shoot a string, you can see an increase in velocity. "
Later in the post you state: "if you remove enough bore friction, chamber pressures drop, burn rates drop, and velocity drops."
This seems, at first impression, to be a conflict.
Best Regards,

IS - With bore fouling, you are not sufficiently altering chamber pressures. All you are really doing is reducing a little bore friction. We are talking about 30 to 75fps difference between a cold clean shot and the 3rd, 4th, and 5th shot as the barrel fouls. This is the increase in velocity which I was referring to. And this seems to happen with most barrels. You will generally see more of a velocity spread with rough factory bores than you do with lapped (smooth) premium barrels.

With moly, you are reducing bore friction to a level where you significantly alter the chamber pressure, burn rates, and velocity. For instance, with 6mm sized bores, I have seen velocities drop off as much a 150fps or more with moly (all other variables the same). The way that you know your bore has been properly burnished with moly is to shoot across a chronograph while monitoring velocity drop. When velocity bottoms out (about 15 shots) you know that the bore is burnished.

One of the best gages for load development consistency is to monitor velocity SDs. If you can maintain single digit SDs, then you have found the internal ballistic accuracy curve. This does not necessarily translate to external ballistic accuracy, however, you know that your loading habits are correct. If I have a bore that shows a large velocity variance between the cold shot and fouling shot, then I throw out the cold shot data when calculating velocity SD.

Hope this clears up my previous post.

ZeroDrift,
Yes, I now see the point you were making.
Thanks