Also rearward force is lessened by molecular adhesion of the case wall to the chamber wall. That is why you can get a case head separation if there is any slop in the mechanism / bolt lockup. The case will stretch anywhere there is not adequate adhesion to stop it.
I shoot straight walled cases in my BPCR rifles and had a rough chamber (I polished it out). You could see the radial expansion due to the marks on the case and there was difficulty extracting the cases, as the brass tried to "stick" to the chamber.
Also in BPCR rifles if there is any moisture in the chamber (people use "blow tubes" to keep the fouling soft) and the case cannot adhere to the chamber walls, sometimes the bullet will grab the case and rip the front part of the case off.
High pressure always tries to flow to an area of lower pressure in the most direct route. The most direct route is out the sides of the chamber. So everything expands. If the barrel (or barrel extension) and receiver are strong enough to contain the pressure, the next shortest path is back through the receiver. If that is strong enough then the last remaining path to drop the pressure differential is shoving the bullet out the front of the barrel.
A "rung" chamber is simply a prelude to a failure that didn't happen because the pressure was released in time.
Also it is not "recoil" that is locking the mechanisms together. It is pressure. "Recoil" is simply the equal and opposite reaction to the projectile (that is entire projectile mass, bullet, gas, unburnt powder, in BPCR rifles, the under bullet wads, shotshell cups, etc).
Remember in school we were taught about every action having an equal and opposite reaction ? Recoil is the opposite reaction of what exits the end of the barrel.
Last edited by drcook; April 20, 2013 at 09:56 AM.