Buck's guns are, for the most part, fairly standard--but they get the job done. His handgun of choice is a Colt SAA with a blued 5 1/2" barrel and walnut grips. His rifle is a Winchester 1873--the "gun that won the west". Although he is yet to use it on the show, Buck also owns a sawed-off shotgun.
Winchester Model 1873 rifle
The 5 1/2" barrelled version of the Colt Model P is refered to as the Artillery Model (it debuted in 1875), the original 7 1/2" is the Calvary Model. In 1878 Colt introduced a civilian model with a 4 3/4" barrel. 1878 was also the first year the Single Action Army was offered in .44-40, which allowed shooters to use the same ammunition for both their Winchester rifle and their Colt revolver.
Buck finally used the shotgun during the episode WAGON TRAIN, and in NEMESIS, J.D. can be seen using it during the saloon shootout.