if the comp is attached to the barrel, it adds mass to the reciprocating assembly
Absolutely. The mass of the compensator does come into play. As does the gas hitting the compensator baffles. Like you say, if it CAN affect it, it WILL.
Slide mass, barrel mass, recoil spring rate, and barrel pull from bullet drag or a compensator all play a part in the complex balancing act that is a functional pistol. Change any one factor, and you have to account for it elsewhere, or there are consequences. (failure to function, destroying brass, abusing the gun, etc)
The other point is that the recoil spring doesn't have a lot of effect in delaying the slide at the very beginning of the cycle
Still right there with you. I'm just trying to illustrate a point about bullet drag / barrel pull, which I'll get to in a second.
By the time the action has unlocked, the bullet and the gases propelling it are out the barrel and gone. The remainder of the slide's travel is powered only by inertia. It's during this part of the slide's travel that the recoil spring comes into play. The spring slows the slide down as it gets compressed. Too low a spring rate, and the pistol gets battered and wears prematurely. Too high a spring rate, and the gun may not cycle.
Now, given the same round firing in the gun, there is a fixed amount of energy being produced. The more energy required to get things to linkdown & unlock, the less energy left over afterward for the slide to keep moving. Increasing the barrel weight, bullet drag, or forward thrust from a compensator require more energy to move the barrel back to linkdown, so the slide will have less energy once the gun is unlocked. You then need to lower the recoil spring rate to balance things out.
Of course, it is theoretically possible to add so much weight and pull on the barrel that the recoil spring required to balance things out is too weak strip rounds from the magazine. It all goes back to that complex balancing act.
But I digress.
Consider that the standard recoil spring for a 38super is 14#. Conventional wisdom says to run a heavier spring with hot ammo, so a gun shooting +P+ should probably have a 16-18# spring, if not more. Yet by adding a compensator to the equation, we can cut the recoil spring rate
in half.
To make a long story short (too late!), here is my point:
We know that bullet drag is significant enough to operate a blow-forward design. Given what the compensator example shows, how could bullet drag
not have been accounted for in the design?
With that good of a picture, he had to of shot the @!#$#% out of it.