Miller,
I have never recovered a LR solid from the body of a squirrel. That is because they always penetrate the squirrel, and expend whatever excess energy is required to do that task on the other side of the squirrel, in the atmosphere. If your .22 LR round produces 30ft pounds energy, but, it only takes 15ft pounds for the bullet to penetrate the squirrel, 30 ft pounds was not delivered to the squirrel. 15 ft pounds was.
The .22 short I almost always recovered from the carcass of the squirrel. It was moving slower to begin with, and because of that fact, and the fact the barrel rifling was cut to optimize the velocity of the higher velocity LR bullet, the .22 short was less stable in flight ( your experience has already alluded to this fact with your Mossberg rifle) than .22 LR, and, the bullet was far less stable in a squirrel, when it got there. It has a greater chance of delivering all the energy to the squirrel, and will do just that if it does not penetrate.
This is not carved in stone, and much will depend on the range of your shot. I never got shots much closer than 30 yards, and if all your shots were at 15 yards, I doubt you would see any difference at all. Somewhere in there is air temperature as well, I never hunted until well after the first frost, and preferred to have snow on the ground as it seemed to eliminate the sick animals, as well as reducing fleas, mites, ticks, and sundry other pests that make their living on the backs of warm blooded game animals.
Best,
Ted
