In terms of hunting with airguns,
Beeman's published an article quite a few years back addressing this, with their recommendations of appropriate minimum power levels to hunt various critters - here's a link:
http://beemans.net/field%20use.htm" onclick="window.open(this.href);return false;
According to their chart, a squirrel requires about 3 ft. lbs, with an impact velocity of ~415 ft/sec for a 7.8 grain .177 pellet.
Probably a reasonable guideline, but IMHO it's like the 1000 ft. lb recommendation some gunwriters say is required for deer - there's an awful lot of "hand waving" in the numbers. (And since a .357 Mag handgun is a reasonable deer cartridge at close range, it's clear that 1000 ft. lbs. is not really needed.)
