Donkey said:
Thanks Tony.
Those are some very interesting points you raise. If what you suggest about the pressure is true then it would follow that the only way to get consistent higher velocities is with a longer barrel. Do you agree? I see you seem to be saying that the main reason for group widening at very long ranges is projectile related. That is my hunch as well. Do you think that the BC variation in projectiles at longer ranges is because of the velocity delta over the course of the shot or that the front of the projectile becomes more important as it slows down or both?
With regards to pressure do you think it could be related more to barrel distortion than case primer limitations? If so a bull barrel might be more accurate at those high pressure nodes. I'm guessing you are running a heavy barrel on your 7mm so may be not unless it is the meat around the chamber that is important rather than down the barrel.
I idea of a pressure limited system sure is a good advertisement for progressive burn powders.
Thoughts anyone?
I think you have raise a number of interesting questions with regard to pressure. I was refering to the process of combustion within the case. I believe this process proceeds differently at pressures that are both higher or lower than the range of pressures at which it is normally used in the cartridges it was designed for.
I do use heavy barrels on my rifles, most straight 1.250 diameter and 30 or 31 inches long. Accuracy has been pretty insensitive to heat. The last 5 shots typically ahve about the same vertical as the first 5 in a string of 20 in the cartridges I have used, but I have heard about scores falling apart at the end of a string with some of the big magnums and I have observed it once my self when I was scoring another shooter. This person was clean through 15 at 1000 in perfect conditions then proceed to drop 8 points in the last 5 shots. I am still trying to understand what actually cause the accuracy to go away but I am reasonably sure that temperature is involved somehow. And I have seen 22 inch 308 sporter barrels start to change POI after just 3-4 rounds.
But you raise some interesting questions about the strength of the actual components such as barrels, actions, etc. I have had some interesting results with one of my rifles, a Rem 700 SA converted to single shot with a nice laminated walnut prone stock that I have adapted for F-Class. I bought it used from a friend with a 31 inch Krieger Heavy Palma barrel chambered in 6mmBR. This combination could be tuned to deliver vertical accuracy that a benchrest shooter would envy but it drifted a lot in the wind so I had it made up as a switch-barrel with another 31 inch Kreiger, this time a 284 Shehane and a straight 1.25 inch cylinder barrel. This thing shot vertical comparable to the 6mmBR but drifted a bunch less in the wind that the BR did. That and another just like it, except with a Nesika action, are what I shoot most of the time.
Then I put a heavy Kreiger 28inch 7mmRSAUM barrel on it and started to see what could be the effects you are thinking about. This combo is a hammer up to a little over 2900 fps with the 180s them starts to fall apart when the velocity gets up to the next node at around 3000 fps even though this is not a maximum load for that case and my chamber. I have wondered whether that big 0.532 diameter case head and the higher operating pressures of the RSAUM does exert too much pressure on the action compared to either the Shehane or the 6mmBR both of which have a .0473 inch 308 boltface and operate with several thousand psi less pressure. I'll know soon enough because I have a 7mmRSAUM on a 1.550 inch round BAT M coming soon, and if that won't stand a lot of pressure I don't know what will.
I do like long barrels because I have been able to tune them without any difficulty and the shorter barrels I have tried needed just as much tuning so why not operate at lower pressures with the longer barrel. This would probably not be true with something like a straight 1.500 diameter barrel in a barrel block. My gunsmith has something like that on a heavy BR gun and he says it isn't particularly load sensitive but it also weighs over 100 pounds.
The variation in BC I was refering to is due to lack of uniformity in the physical dimensions of the bullets. It is not unusual to find ES in overall length of as much as 0.040 inches in a single box of 100 bullets but ES of 0.020 inches has been quite common in my experience and I have measured many thousands of them. Sorting into groups the same length can improve vertical dispersion. Trimmed to the same length then pointed improves it more.
But you raise interesting questions about how BC might vary as the bullet travels down range and looses velocity. Clearly, BC is a function of velocity although the curve is not a straight line. But it does change more in the zone between the speed of sound and about 1.5 times the speed of sound than it does above those speeds. This zone seems to be particularly relevant to your original question because the 155 gr 308 bullets do drop into that region as they near 1000 yards. Byran Litz recently talked about stability in the transonic region in a post on the Long Range forum. You might find that interesting reading.
