Y'all are forgetting the not-insignificant force of the actual hot gases themselves.
Not at all...but the residual recoil effect of the gasses exiting behind the bullet is fairly insignifigant in a straight-walled pistol case with the typically small powder charge. And...By the time it comes, the recoil-operated pistol's slide has already gotten all the momentum that it needs to cycle.
The mass of the gasses are roughly equal to the unburned powder charge.
The .45 ACP round, for instance...typically uses a quick-burning powder charge of about 5 grains and exits the muzzle at or just slightly above the speed of sound. We'll say a thousand fps just to keep it simple.
5 grains of mass at a velocity of 1,000 fps would generate about 1/8th the recoil impulse of a standard velocity .22 Short. In other words...not much.
Bottlenecked rifle cases are a different matter, and the impulse generated by the exiting gasses and unburned particulate can create an after effect that the bullet itself provides...and in some rounds, it can actually exceed it.
The shoulder and neck create a venturi effect, which speeds up the exiting gasses to exceed the velocity of the bullet. Coupled with the mass of the exiting gasses, the recoil impulse that is generated can be signifigant. Figure up a 39-grain powder charge exiting at nearly 5,000 fps with a 45-grain bullet fired in a .22 Swift that exits at 3800 fps, and the "aftershock" can easily exceed the primary recoil impulse generated by the bullet.
But in a straight-walled pistol case? Wouldn't disturb a fly.