Not a .45 bullet, but a .44 bullet. Stupid reporter. .45 caliber didn't exist in the 1860s and is never used at re-enactments. The guys was probably shooting either an 1851 Colt or an 1858 Remington repro pistol.
Change the barrel and the cylinder, and it can be anything you want it to be.
Personally all my black powder weapons are 45 caliber, so that I have no problem loading a 50 into a 45 or a 45 into a 44.