[ceacar99]

welp as i said in my intro im a small arms technician seeking true knowledge about firearms, down to the core of matters, not just how to figure out whats wrong and replace/repair a part.... in my studies(and i seriously look far deeper then made my instructors comfortable) i came up with a few questions that i realy just cant find an anser for, tonight i want to focus on this....

locking, why? even a thompson smg had a special locking system put into it so that it wouldnt open at the height of the pressure from the round, but rather when that force ebbed. does locking the chamber prevent explosive acedents? ensure that the buffer(or mainspring) is not over stressed? or maybe some realy easy to spot thing that i didnt see. keep in mind, i know how to repair these things but im trying to get at the heart of how they work and why they were built that way.

 

[c3shooter]

Lowered powered autos are "locked" by the strength of action springs- they are "blowback" designs- breech held closed long enuff for bullet to leave barrel. Usually found in 22s, 25s, etc. More potent ammo will require true locked breech- 1911AI has lugs where barrel locks to slide, until it has recoiled 1/4 inch or so, then link pulls barrel down, and it unlocks- allowing slide to move away from breech of barrel. FWIW, the Blish lock of Tommy Gun fame was pretty much found to be uneeded, and later SMGs were blowback. Strong spring.

 

[1hole]

ANY action that is unlocked at high pressure can be considered to have blown up.

 

[CARNUT1100]

Another factor is that if the slide starts to move before the pressure has gone down a little, then it will rip the head off the case.
There is an incredible amount of presure on the walls of the case gripping the chamber, while the cse wall is fairly thin.
Notice in rifles with excessive headspace the case stretches, as the head is blown back to the breech but the case walls stay where they are until the pressure subsides.
Eventually the cases thin near the head and break off if they have been reloaded too many times.

All firearms do it to some degree, but hotter loads or greater headspace accellerate it.
If a semi blew open at high pressure you would rip the head straight off and let a lot of hot gases come straight out the back at high speed, with potential for damage and injury.
Not good.

Just as a quick example, using a 9mm cartridge,
the area of the cartridge wall is 537mm squared.
Assuming a coefficient of friction of 0.3 ( cartridge brass on a smooth chamber) and a peak pressure of 35000 psi or 241 MPa;
the gripping force will be the pressure x coefficient of friction multiplied by the area, giving a figure of 38,873 Newtons of force.
The breaking stress of brass is 550MPa.
Assuming a wall thickness of 1mm, the sectional area will be 25.13mm squared. ( I think the wall thickness is smaller, but have assumed 1mm to be on the safe side)
Therefore the stress on the cartridge wall iff the head is unsupported will be 1,546MPa before the case wall will pull out of the chamber ( stress being force/area)
This is 3 times the breaking strain of the brass, so the head will come of before the case will move at peak pressure.

The force on the head at that pressure is the pressure x the case head area, which comes to 15350N.
The stress on the case wall will be this force divided by the case wals section area, giving a stress of 610MPa, so with a rupture pressure of 550MPa, there will be enough force on the case head to break it off.
However, if you delay it just long enough for this peak pressure to decay a little then it will not rip the head off.

Sorry for the quick and dirty calculation, and not showing all of my working out, but it would have been too messy to type it all out.