500 Nitro

DRs barrels attach to the action on a pivot (open the action and the barrels swing up and away fron the action face), so any force acting to drive the barrels away from the action face will force them around that pivot, up and away from (off) the face.

Conjecture aside, the question really is about the term backthrust, this appears to be discussed as a phenomena distinct from pressure. I'm trying to figure out what this backthrust could be that is DR specific (the pivot action?) and referred to as both distinct from pressure and seems to be able to be calculated.

AR2201
Those who can make us believe absurdities can make us commit atrocities