Do you have a picture of this firearm?
It's hard to tell without a close look and the man's size is throwing me off - I know he's a giant. It looks like a New Army Colt (aka Peacemaker, Frontier, etc. and several other names) with a 8 1/2 inch barrel. Considering his size though, it could be longer. They were available in several barrel lengths up to something like 10 inches. The so called Buntlines were all special order up to, I believe, 16 inches and could include a stock attachment such as was available on the old Dragoon models.
The standard "New Army" or "Colt Model 1872" was first manufactured in 1871 and introduced in 1872 in .45 cal. with a 7 1/2 inch barrel. Shortly after (and following an 8000 order from the US Army) it was released in the same size but in .44-40 Winchester so that those using them could carry one cartridge for both pistol and rifle. However, in Walker's hand, this one looks longer.