I understand, Sylvan...but you still haven't wrapped your head around the fact that...while the 180 may indeed have more momentum at exit velocity...it may not necessarily be the case at the beginning of the event, where the majority of recoil occurs. Muzzle velocity has very little to do with what you feel as recoil, and...in an autopistol...you feel nothing from the actual internal ballistic event. By the time you feel it, the action and reaction between the bullet and the breechblock is over. Done. Finito.
What you feel comes from the slide compressing the spring and its impact with the frame. If you've watched the videos, you've seen it.
In order for the slide to impact the frame harder, it must have more momentum. In order for it to have more momentum, it must be moving faster at impact. Must be. Momentum is a function of mass and velocity.
In order for it to have more velocity at impact, it must have started out faster...during the peak pressure and force imposed on it. That occurs at about a half-inch of bullet travel, or less...just as the bullet hits the rifling.
It's at that point in the sequence that peak force is imposed, and it's at that point that about 95% of the recoil impulse is generated...and you don't even feel it yet. The slide and bullet do...but all you get is a little push from the spring...and it's at that point that the speedy 135's momentum is greater...even though it may not be at exit.
Then note how much rearward motion of the slide is needed to unlock the gun.
Thank you, Vern!
Unlock begins at about 1/10th inch of slide travel. The bullet must be gone, or at the point of leaving before that can happen. By 1/8th inch, the breech is open, and even the residual gasses are gone, and can't impose any more thrust on the slide. The slide can't accelerate any faster after that happens any more than a bullet can accelerate after it leaves the muzzle.
Sylvan...Read that again. Once the bullet exits the muzzle, there is no more recoil. The action/reaction event is over. The bullet and the slide are moving because of the momentum that they conserved during the event.
Now, back to the bullet for a minute.
The bullet is resisted by the frictional forces imposed by the barrel. An object of lesser mass loses momentum at a faster rate than one of greater mass...so the 135 bullet loses momentum at a faster rate. The frictional resistance is constant.
Now for the slide and spring.
The slide's mass is constant. The spring's rate is constant. The slide's loss of momentum is controlled by the spring.
In firing the 135/1180 load, the slide is moving faster at the moment of peak force...pressure. It has to be in order to match the bullet's momentum.
Now for an analogy that may help.
Fire a .30 caliber bullet at 3,000 fps muzzle velocity and a 180 grain bullet at 2800 fps MV. At 100 yards, the lighter, faster bullet has lost a greater percentage of its initial velocity than the heavier one. By 300 yards, the 180's speed is roughly equal to the 150's. By 400 yards, they're neck and neck...the 180 is closing the gap. At 500 yards, the 180 overtakes the 150 and passes it.
It's actually moving faster, even though it started slower.
Why?
Two reasons. Momentum is the obvious answer...even if they started out at equal momentum. The faster the bullet is moving when it hits the air, the harder the air fights it...offering it greater resistance than on the slower bullet...even though the atmospheric pressure is identical.
The faster any object is moving when it encounters an outside force, the faster that force will slow it down.
So, forget momentum calculations at muzzle velocity, and consider what happens while the bullet and breechblock are compelled by peak pressure and force.
To recap:
In order for a given load to produce more felt recoil than another one in an autopistol, the slide must hit the frame with more momentum. Because momentum is a function of mass and velocity, it must be moving faster. If it's moving faster, it's compressing the spring faster. When the spring compresses faster, it pushes backward on the frame faster.