Rifle Case Shoulders
Date: Feb 01 2007
Modern theory has it that the shoulder shape of a rifle case (whether shot in a rifle or a hand carbine) has a great effect on both accuracy and "efficiency." Supposedly cases with steep shoulder angles, anywhere from 28 to 40 degrees, help keep the powder in the case, where it burns more completely. Thus the "magic" of the recent short-fat-beltless magnums, allowing them to match the muzzle velocities of bigger cartridges with less powder.
I wish this were so, but all indications so far show that cases with the same powder capacity produce the same velocities at the same pressures. In fact, in the present edition of Handloader magazine (No. 244, Dec. 2006) I describe an experiment by Texas riflemaker Charlie Sisk, where he chambered the same barrel for the .300 WSM and .300 H&H, two cartridges of almost exactly the same case capacity but very different shapes. When Charlie shot the same loads in both chambers, velocities and pressures were pretty much identical.
So was accuracy, but many other experimenters do feel that the shortfat- beltless magnums are more accurate than longer, belted magnums of about the same capacity. Accuracy is a more slippery devil than pressure and velocity, since it varies widely with the quality of the barrel, how well that barrel is set up, and of course, the ammunition and shooter. So I am still waiting to be convinced one way or the other about accuracy.
Shoulder shape, however, does have some effect on other aspects of handloading. One is case stretching. A steep shoulder of 40 degrees or close to it will just about stop case stretching with normal loads, evidently because the steep angle keeps brass from being pushed forward both during firing and during case sizing.
With standard cases, stretching during firing is caused by a relatively shallow shouldered case being driven forward slightly into the chamber, like a wedge. After ignition, the case head backs up slightly, stretching the case. This is the reason that cases with very shallow shoulders have a reputation for separating just in front of the case head after a few firings. In contrast, a steep shoulder holds the case firmly in place, and no stretching takes place.