That gas jet is what generates felt recoil too. When the bullet clears the barrel that gas hits the amosphere and pushes the rifle back into your shoulder.
Hey filaman,
It might have an effect on "felt" recoil but it hasn't got much to do with mechanical recoil. By felt recoil, I mean you might "feel" or believe (big flash, maybe) it's affecting you more when in reality, it's not doing anymore than any other load of the same recoil energy level.
Mechanically, that jet doesn't rely on the atmosphere. If that were the case, spacecraft wouldn't function very well in space since their exhaust wouldn't have anything to push against. Instead, it's all about Newton's Third Law. Others above have already alluded to it so there's no need to rehash it here.
I also agree, exit pressure at the muzzle, the direction of that exit pressure (specific brake), the nature of the expansion wave play (what kind of "flashhider/thing on the end") or even the sheer volume of the pressure wave (artillery/tank) will play the biggest role in how loud something is or perceived.
Additionally, whether the shooter has good hearing or is partially deaf plays an important role. 😁
