In the comics it's a .44 Walker Colt and they are magic and so not subject to earthly physics.
Colt Walker at Internet Movie Firearms Database
The Colt Walker, sometimes known as the Walker Colt, was a single-action revolver with a revolving cylinder holding six charges of black powder behind six bullets (typically .44 caliber lead balls). It was designed in 1846 as a collaboration between Captain Samuel Hamilton Walker and American firearms inventor Samuel Colt.
The Devil tended to the gunman's wounds, the magical Walker Colts were cast, and the man left Hell—but not before gunning down the Devil after the latter insulted him.
Wikipedia - Saint of Killers
The Saint of Killers possesses a pair of Walker Colt revolver pistols, which were bequeathed to him by the Angel of Death, composed from the previous Angel's steel sword and forged in the last fire that burned in Hell, because of the nature of their making, the guns are demonic in nature:
- Any shot fired from these guns will always hit its mark, even piercing modern tank armor to reach the intended target.