AR2201

"Upon firing a cartridge backthrust acts to open the locked DR mechanism."

Where do you get this from ?

When a gun is fired the majority of the force is trying to move the barrels
away (forward) from the action - ie the force is a predominantly stretching

I admit that some of the force will act in a pivoting way trying to open
the gun but not much compared to the other force.

Why don't you get a DR or Shotgun, cut all of the locking bolts
and 3rd bites off and fire the gun. That should answer your question.

AR2201 - are you an engineer or a scientist by any chance ?
(Just asking from some of the questions you ask)

500 Nitro