(The opinions expressed in this document are personal and should be verified as necessary)
There are three main calibres .22 .38 .357
.32 handguns are not widely available.
For small animals - .22 calibre is adequate
The .22 bullet is extremely fast - the muzzle velocity is between 1050 and 1435 feet per second.
The weight of the bullet though is only between 32 to 40 grains.
For larger animals the .38 and .357 are more suitable.
A .38 handgun will only take a .38 bullet - whereas the .357 will take either .357 or .38 bullets.
The .38 is more suitable for medium to large animals, and the .357 is more suitable for very large animals i.e. bulls.
With a .357 handgun you would have the choice of which bullet to use.
The .357 revolver is normally built on a larger frame.
An application for a humane killer in these calibres should read .38/357 - 2 shot revolver.
The .38 bullet has a muzzle velocity of 710 to 1250 feet per second and a bullet weight of 110 to 158 grains.
The .357 bullet has a muzzle velocity of 1145 to 1542 feet per second and the bullet weight is between 110 and 180 grains.
A comparison of handgun bullets is shown below:- (not actual sizes)
Left to right: 1) 3 inch 12 ga magnum shotgun shell; 2) AA battery (for size comparison); 3) .454 Casull; 4) .45 Winchester Magnum; 5) .44 Remington Magnum; 6) .357 Magnum; 7) .38 Special; 8) .45 ACP; 9) .38 Super; 10) 9 mm Luger; 11) .32 ACP; 12) .22 LR