Because even gasses have mass. When you apply a force to accellerate any mass, there in an equal force applied in the opposite direction. Think jet engine.

As for when recoil begins, it begins the instant the mass begins to move. The effect of the gaseous mass accellerating down the bore is also felt immediately because the gas accellerates with the bullet.Also remember that while thegasas a whole may not be moving very fast while the bullet still contains it in the bore, the individual gas molecules are in a veryexcited state becauseof the high temperature and are moving veryquickly.Pressurequite literally is theaverage kinetic energy transfered bythe impact of moving molecules with the surface of an objector containerthey interact with.

Mike