It could be like Wayner said and the hand spring is exerting enough forward pressure on the cylinder to close the gap. I wouldn't be too concerned unless it binds when you shoot it. Heck, you might have a navy-nagant with a cylinder which seals on the barrel! Just joking. If it does bind, maybe you could clean the front bearing surface of your cylinder arbor well and apply some epoxy, allow it to harden with the gun apart, then sand it off a little at a time until it's just the right overall length. If you measure it's length when it fits right, I'd think you could knock that epoxy off there and add metal with a wire welder, then file that down until you get the correct measurement back. A real gunsmith like Wayner and some others on here are probably capable of fixing a short arbor better than that way, but if it was my gun and I had to go at it alone, that's what I'd do.
Steve
Steve