You couch the question in a funny way. Do you mean, what is the ratio of the bullet mass to body mass? And if so, is it a relevant comparison? I doubt it. It would probably be a better comparison to look at energy, not mass, and not just energy, absorbed energy. That is, a projectile that stops in the larger, thicker tissue of a man delivers all of it's energy whereas a projectile that passes through a small varmint delivers only a fraction of its total energy to the target
Here is a rough estimate.
150lb man vs 1lb squirrel = a factor of 150X mass
40 grain (22LR) bullet vs 8 grain pellet (.177) = a factor of 5X mass
40 grains @ 1250 fps =139 FPE , 8 grains @ 900 fps = 14.4 FPE, or a factor of 5X mass and 10X energy in FPE between a typical 22LR and .177 air rifle (energy = 1/2 mass x velocity squared, so energy goes up proportionately to the mass but increases as a power of 2 to the velocity.
150 x the mass of the 8 grain .177 pellet at 900 fps would be 1200 grains, 1200 grains at 900 fps, or 2159 FPE
a 7mm Rem Mag with 185 grain bullet at, what, say, 2500 fps (guessing) would be 2568 FPE
so, maybe one could make the comparison of whacking a 1-lb. squirrel with an 8 grain pellet to shooting a 150-lb man with a 7 mm Rem Mag, using proportionate mass as the factor, but I doubt that is a "fair" comparison.
