Craig;
The total recoil energy is determined solely by weight & velocity.
Where the "Felt Recoil" comes into play is based on rate of acceleration, The quicker the acceleration, thus shorter barrel time to achieve that velocity the "Sharper" the recoil. While I in no way question this concept I do have some reservations concerning all the "Hype" sometimes published on Felt Recoil.
The big deal here is as you mentioned "What amount of change in time can be perceived as different".
While I do not have any concise answers on this I do have grave doubts that within the parameters of a powder of suitable burn rate for a given shotgun load that there truly is a "Feelable" difference in actual acceleration rates.
For instance to load a 1 oz shot charge to 1200 FPS powders in the range of Red Dot to Green dot are going to be suitable, regardless of brand name on the can. Start going to much slower powders then you are courting squibs & failures.This is particularly applicable if you are going to use the loads when temperatures may fall down in the below freezing range as in hunting loads. For strictly Hot weather use one does have a bit more lee-way but I still do not like these Extremely "Low Pressure" loads. I realize many here will disagree with this but this is using those powders in a way for which they were not designed nor intended.
