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.
Umm, no. It is not the "gas jet hitting the atmosphere". The recoil is a simple conservation of momentum.
(Mass of bullet x its velocity) + (mass of propellant x its velocity) = (mass of rifle x its velocity)
The rifle starts to move as the bullet starts to move. It is actually starting to recoil before the bullet leaves the muzzle. However the propellant exiting the muzzle, and due to overpressure doing so at a greater velocity than the bullet, certainly plays a significant part, simply by reason of the momentum it contributes to the system. It is nothing to do with "hitting the atmosphere and pushing" though, and would contribute to the recoil in vacuum in the same way.
The mass of propellant is often overlooked in thinking about recoil, and it is possible to have two loads with the same bullet at the same velocity, but significantly different recoil, if you change from a slow powder (and hence greater mass of powder) to a faster one (and hence less mass).
Of course the gas Mxv is vectorial too, and so you can also reduce recoil by redirecting it, such as into a gas piston, or via a brake or can.
