Home Forums Hunting & Shooting Ask The Gunwriters What difference is there with breach pressure and chamber pressure?

Since it's the powder gas that's exerting the force, the flat portion of the inside surface of the web (exclusive of wall thickness, etc) is the active piston surface. Using the diameter of the rim produces an area figure that is too large.



In lieu of direct measurement, I methodologically estimate the area of this net surface by using 90%-95% of the outside diameter. This allows for the fact that the case wall and the fillet (curved transition from web to wall) are not subject to the straight-back force.



Peak chamber pressure (lb/sq in.) times this net web area (sq in.) equals the force (lb) pressing the case rearward against the resistance of the locked lugs.



Obviously, actually measuring the diameter of the flat area of the web produces a figure that is more accurate than my hasty estimate.