Oh, yeah. Or picked up from the photographers prop table, a common practice when having their picture taken off duty.
There is only one standard issue type revolver shown in the three portraits, that being what appears to be a Colt M1851 in possession of the fellow in the second picture, and it has a stock, a rare item indeed in the military since they were tested by Ordnance and found to be dangerous to use because of the side flash from the cylinder/barrel gap burning the shooter and they were useless (in the way) when used by mounted cavalry. All others are civilian small caliber pistols that could have been the soldiers' "spares" or, more likely, were owned by the photographer for when one of his clients wanted to look a little more ferocious than usual.
The photos are very interesting and there is no doubt that they are original and taken during the period, but the first one brings a smile to my face. Have you ever tried to ride a horse with 4 revolvers stuck in your belt like that? Much less use them. Sorry, no offense meant, maybe the young fellow kept them in a saddle bag or in the unit transport in the rear. And I do find it interesting that none of the three seem to have the standard issue holsters required to carry the Colt or Remington revolver they were issued.