lysanderxiii
Member
While The case cannot be stretched as the bolt closes, depending on the type of action, tolerances of the bolt and barre, and the chamber headspace, the case stretch can be exacerbated, It is very common in machine guns, but also seen in M1s, M14s, and FALs. It happens when there is excessive space between the front of the bolt locking lugs and the forward arresting face of the barrel.@Slamfire how does brass get stretched as the bolt closes?
When you check the chamber headspace, you measure the distance "A", but normally you do not know "B".
So, when a semi-automatic (or automatic for those lucky ones) chambers, the bolt stops when it hits something firm and unyielding, which the case is not. The case will get resized to a new length.
Slamfire's description is generally correct, however the friction of the primer in its pocket coupled with the action of the main spring and hammer will hold the bolt forward with enough force to keep the primer from moving rearward. While the pressure in the case is 50 to 60 KSI, the pressure has to flow through the vent and reduces the pressure, as well as the area available is far less. Tests measuring primer pushout forces show it to be around 80 to 95 pounds for normal chamber pressures. The force required to push a properly fitted primer in a case 90 to 100 pounds.
If you are seeing primers pushed out of their pockets, that is an indication of case deformation due to high pressure or high bolt thrust. If the primer pocket gets loose you run the risk of bolt face erosion.