to the OP, I'm not expert - but, in your description of what pulls the barrel, I think the round sealing against the chamber and the pressure of the gas as the round is fired, creates the greatest force backward on the barrel/chamber when the round is fired. Is this offset by the drag of the bullet in the barrel pulling it forward, never worked through the exact timing of how that all works from a science thinking perspective. If I had to hazard a guess, most guns would probably be fine if the barrel is press fit and the pin is there as much for alignment as to secure it. Eventually use and recoil might work it loose, but like old revolvers have pinned barrels, and some of the new ones of the same model, now they don't have pins because they decided not needed. Although, now sometimes revolvers get sent back because the barrel is a little canted/tilted, and the pin would prevent that QC issue.
