Uh, not an expert, Wonk, but, your analysis seems a bit light on the law of "conservation of momentum", that being a lighter object exhibits greater impulse than a heavier one receiving the same amount of energy. If I load my sub 5 lb. Italian folding 12 gauge ( Relax! we're talking a 3" chamber here-who says the Italians are cowards?) with one of your pigeon loads, I'm pretty sure it'll bend your remaining teeth in a way the P gun might not.
Only thing I can surmise about amps is, I'm betting you own several....
Best,
Ted