by Wyliecoyote » 18 Mar 2024, 3:51 pm
Cartridge cases are a pressure vessel, not a cylinder head exhaust port, the bullet is the plug and the bore is the gas escape vent. Pressure is pressure no matter what the case geometry might be. In bottle neck cases the shoulder angle has a direct bearing on brass flow where the steeper angle generally has less flow and vice versa. A case that has 3cc volume, no matter what its shoulder angle might be, filled with powder will produce the exact same pressure as another case of differing geometry but with the same 3cc volume. The other thing about shoulder angles is said to be that steeper angles extend throat life. I know that not to be true because the 6 Dasher destroys throats in way less than a thousand rounds as does the 7mm SAUM. A Dasher has a 40 degree shoulder angle, the parent 6BR has 30. The Dasher uses 2 grains more powder than the the 6 BR with the same bullet weight, has a 30% shorter neck yet the 6BRs life span is in excess of 5000 rounds. What greatly extends throat life is neck length and that is a given irrespective of shoulder angle. The further you can keep the heat away from the throat, the better off you are.
Where efficiency comes into play is case volume paired with the right powder burn rate. That is where the real development has been over the years with so many powders to choose from. In specific regards to the 6.5 CM with 140 grain bullets, i can fill the case with 2209, damage it after one firing, and not achieve anywhere near the velocity i can with less pressure from Alliant Reloder 26 and a case that is hardly stressed at all. So the Creedmoor case is not the efficient part, the powder used is.