Beautifully done video!
Hi, All - Being an engineer, I wanted to throw in my 2-cents-worth on this discussion. I have to confess, this is just my theory, since I've only shot hand guns that do not have mechanisms (such as sliders) that absorb recoil. See what you think...
The recoil does, in fact, begin the moment the bullet first feels the force of the propellant, and as a result, the rear end of the barrel (face of the breech plug in my flintlock) immediately feels an equal and opposite force that tries to push the barrel and stock backwards, thus causing muzzle rise, because the pivot point in my wrist is below the center line of the barrel.
In the case of the pistol in the video, that force immediately begins pushing back on the slider, not the stock. Since the slider slides, there's not yet much recoil force on the stock. Then at some point the slider reaches the end of its travel (also having compressed the slider spring by this time). When this happens, it's the momentum and spring compression of the slider that pushes backwards on the stock and causes barrel rise. But it doesn't affect POI, because by the time this happens, the bullet has already left the barrel.
So that's how recoil begins immediately, but barrel rise doesn't for this type of gun
Now, in the case of my flintlock, or a revolver, neither of which have mechanisms to absorb recoil, the backward force on the stock begins immediately, which means muzzle rise begins immediately, thus affecting the POI.
I suspect this is a large part of the reason modern artillery have built-in recoil absorption mechanisms.