Ref the gas seal, the cylinder does slide forward to cut down (not eliminate) the gases escaping from the gap between the cylinder and barrel.
Weather if really makes a difference is up for grabs. It's not a real powerful round regardless.
What I can tell you about the Nagant is it's a heck of a lot of fun, well worth the $100 bucks.
Mine came with a holster, and some tools. The trigger pull isn't the best but not too bad in single action. Double action is rather stiff.
If you concentrate on the front sight it is a fairly accurate revolver.
In reality, if you count the many purges in Russia, under the Czar, Lenin, and Stalin, I would venture to bet the Nagant Revolver killed more people then any pistol or revolver in history.
It's a good companion for you Mosin. I wouldn't count on the price stay low for long as people start realizing how much fun the little revolver is.
Suckers so ugly its cute.
If you get it, and want to reload, get back with me and I'll give you the info I developed.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071