Just at the point where the wad is ready to exit the gun's muzzle the bore will be "Full of Gas", thus you could say half of it has moved the length of the bbl. If we take a 1 1/8oz load with a 35 gr wad & 24 grain powder charge, then we have a total wt moved to muzzle of .077lb. Ignoring the fact the wad is somewhere ahed of the breech to start, just figuring on the full length of a 30" bbl & a 7 lb gun, then just as the wad clears the muzzle the gun will have moved .077/7 x 30 = .330". The exit of the gas will give it a little more impetus, but not a lot. In this slightly over 5/16" of movement, acquired in around .003 of a sec, it will have attained it's recoil velocity & be carried on back my inertia.
With "All" other aspects of the gun being equal "Any" difference you can "Feel" can be "Measured" as it will come from either a significant change in either wt of ejecta or velocity of eject or both.
As Jim has so well pointed out recoil pads, gas autos, stock fit etc, which can & do, effect felt recoil are external effects which do not take place inside the bore of the gun.
While we talk of fast vs slow shotgun propellents, in the overall scheme of things they are "All Fast".
Maybe, just maybe, if you could get a charge of 5010 machine gun powder, or similar, to burn properly behind a 1oz load of shot the acceleration rate would be changed enough you could actually feel it. Within the range of suitable powders for a given load, I "Very Seriously" doubt it can be felt by a mere human. To Measure this difference would require equipment which not only measures total recoil, but rate of acceleration (probably doable in a large ballistics lab but maybe not worth the effort at present).
As again Jim has so well pointed out there are a lot of good salesmen out there who play on the "Unmeasured" aspects of recoil.
There are a lot of things which are totally "True" in "Theory" but from a practical standpoint are "Absolutely Insignificant". This is what the snake-oil-salesmen prey on, just a hint of theory that folks don't think through.
