The 10mm cartridge was probably chosen because 9mm Parabellum has traditionally been the European round for 100+ years now even in the face of ineffective stopping power. Honestly, they should have just went with .45 ACP but since almost none of the guns in the game are directly based on real ones, they probably didn't want to do that. The game's pistol lineup should have went something like: .45 semit-automatic pistol, .357 Magnum revolver, and then the Mauser or the Luger (former preferably). Of course, the few other oddball pistols would have been there too.
As to some of the weapons having a cylinder? Got me. The pistol looks like it just has a laser sight on the side that happens to look like a cylinder. You can not mix a magazine-fed pistol with a revolver cylinder like that. If you did manage it, it would essentially be two guns with one grip... Pointless. - Colt