Last week, I took a 1910 Winchester to the range. It's a blowback in .401 WSL. I have shot it a few times with 4 different loads and maybe it tells us something about force and pressure. The breech block is held shut by a big spring attached to a big weight hidden in the fore end. No locking lugs, but it is a strong spring. It's about all my arthritic fingers can do to open the breech and put in the magazine. It takes force to do that.
The only loading data with any reported pressures are found in Sharpe's 1937 Complete Guide to Handloading. Hercules recommended 24.7 grains of 2400 with a 200 grain jacketed bullet. This generates a reported 50,000 psi. With the 250 grain bullet, 23.3 grains of 2400 generates the same pressure. Those loads may generate 50,000 psi, but they do not generate enough force to reliably operate the action. The breech block is not blown back far enough to eject the case, there is unburned powder in the residue in the action, and the rifle is inoperable until the action is worked by hand and the fired case extracted. If you stand behind and to starboard of the shooter firing these loads, you can see fire in the chamber when the breech block opens.
When I switched to IMR 4227 powder, the action operates as designed. DuPont did not provide pressure data but recommended the following: 31.0 grains approximate factory ballistics with the 200 grain bullet and 27.5 grains approximates the factory load with the 250 grain bullet. With either load, hot empty cases come flying out of the action. The recoil spring is compressed far enough to load the next cartridge from the magazine and the rifle operates as intended. If you stand behind and to the right of the shooter, you do not see fire in the chamber.
The loads of 2400 may well develop the reported pressure in a test rifle with a fixed breech. Campbell's book shows a High Wall used by Winchester to develop this cartridge. But the powder burns too slowly to impart sufficient force to operate the action. IMR 4227 burns faster and provides the force necessary to operate the action.
Richard
