An interesting subject, and one that's commonly misunderstood.
If I may...
Felt recoil in an autopistol is a different animal than it is in a fixed breech revolver. You feel very little, if any, recoil impulse from the explosion of the powder charge and the ensuing action/reaction between the bullet and the breechblock. The breechblock...essentially the slide...moves on rails with minimal resistance.
If the rails were 30 feet long, and there was no recoil spring to impede its movement...you could fire a round like the .500 S&W Magnum and literally feel nothing beyond a light disturbance...until the slide hits the impact abutment in the frame. This is where the actual muzzle flip comes from. The actual shock coupled with the altered center of gravity.
The majority of the impulse that you feel in your hand comes from the sudden compression of the spring. The faster it's compressed, the sharper the recoil.
The heavier the spring...the sharper the recoil.
To understand it better, consider that the autopistol...whether straight blowback or recoil operated...is comprised of two separate closed systems, each with its own action/reaction pair of objects, and each with its own force vector.
The bullet, chamber, breechblock and the burning powder charge make up the primary system. The slide, frame, and recoil spring make up the second.
We understand how the first one works. Bang. Force forward on the base of the bullet equals force backward on the breechblock. Both start to move at the same instant.
The second is a little more tricky.
With the slide static, a 16 pound recoil spring offers about 9 pounds of resistance to the slide. On firing, the slide moves nominally 1/10th inch...and the bullet exits. At that point, all recoil from the primary system is over, and the slide continues on the momentum that it conserved during that 1/10th inch of travel. The bullet is gone, and can no longer have an influence on the slide.
As the spring is compressed, its resistance to being compressed increases, and...because force forward equals force backward...the force that it imposes on the frame does likewise.
Moreover...The slower that the slide accelerates, the lower the felt recoil...because the slower it moves, the slower it compresses the spring, and felt recoil is spread out over a longer duration of time. This is why reduced velocity ammunition produces reduced recoil. It simply causes the slide to be driven more slowly...not because its pressure is lower, though that does factor in. More pressure results in more force and more force results in faster bullet acceleration, and faster bullet acceleration results in faster slide acceleration.
Conversely...Bumping the powder charge into the +P category causes the slide to accelerate faster, and because force forward equals force backward...the frame is likewise pushed backward more violently.
Muzzle flip and felt recoil from the spring's influence occur so close together and so quickly, that it's hard to tell the difference...so they're often confused or considered to be one in the same. A heavier spring reduces the one and increases the other...and vice-versa.
