Originally Posted by
Larry Gibson
Me thinks most all are beginning to understand the RPM threshold and what is really occurring. Even Leftiye is on the right track as what occurs during internal ballistics (acceleration) to upset the cast bullets balance (no matter how carefully we cast and loaded it to maintain balance and concentricity) is exactly what the centrifugal force acts more forcefully on as the RPM (velocity with a given twist) is increased. Gear and Goodsteel are all over it because it's what we do to keep the bullet balanced as we increase the RPM/velocity in a given twist barrel that determines where the RPM threshold will be. Using a slower twist, if one has the opportunity, certainly is a benefit as it keeps the RPM lower and the adverse effect of the centrifugal force lower at higher velocity.
357maximum
Yes, I briefly flirted with Edd's alloy before I began the move from Washington to Arizona a year and a half ago (where does the time go.....). I met with almost immediate success boosting the 311466 from the 27.5" Palma barrel at 2600 fps/40,000 psi to almost 2700 fps with 43,000 psi (taking that off the top of my head just for conversation here-would have to research the records for the actual data). Accuracy remained the same with 1.2 - 2 moa 10 shot groups at 200 yards.
The initial problem I ran into was simply I was out of case capacity in the .308W. Faster powders for increased velocity also increased the psi. Slower powders simply did not give even the 2600 fps so while the psi was lower so was the velocity. Solution; a larger case capacity. Hence the interest in this 30x whatever cartridge in the 14 or 16" twist barrel. I believe it can contain enough RL19, AA4350 or H4831SC at close to 100% case capacity to maintain 40 - 43,000 psi and 2700 - 2800 fps out of the 26" barrel with 150 - 160 gr cast bullets.
Note the 26" barrel minimum I am getting for this cartridge; as mentioned muzzle exit pressure is very important. The longer the barrel the higher the velocity can be with less muzzle exit pressure. Several have posted or PM'd me asking about short barrels with slow twists getting higher velocities. The requests have revolved around 18 - 20" barrels but even a 16" barrel was considered. I try to explain that those HV, even with a slower twist, will necessitate faster burning powders, higher time/pressure curves and thus higher muzzle exit pressures. With a proper ternary alloy cast bullet the shorter barrels become a very difficult obstacle to overcome. Just part of all the things we must consider if we are to keep the bullet as balanced as possible to push it to higher velocity before it bumps into it's RPM threshold.
Larry Gibson