Lee has a die set for the 7.62 Nagant, available from Midway, that will load the original brass, retaining the gas seal. I've got one of these sets, works great in my gun. 7.62 Boxer brass is now available from Grafs as well, and you can get a 50 round box of PPU (Serbian) ammo in the low 20s, with boxer primed reloadable brass. I was going to go the .32-20 route, but now its nearly as cheap to just go ahead and load to original spec.
I have been told that if you're having trouble chambering, sometimes the crimp bulge on the case mouth will give trouble. If so, a 7.62x39 Factory crimp die will solve the problem. (Nagant brass is 38.5mm long!)
The Lee die set originaly was set up to load .32-20 brass, and indeed, 2 of the dies are stamped .32-20. But they've very recently modified the seating die post to fully countersink the bullet as per the original, and no longer include the .32-20 qualifier on the cover. I use a Lee mold for the .311 100 grain round nose. Over an acceptable charge of Trail boss, shoots very nice indeed.