Question:
If I start reloading my 7.62x54r and 7.62x39 ammo, does a 7.62 nato (.308) bullet fit in those cases and would it present any problems when fired? I ask because there are some special hunting bullets you can not get in the Soviet cartridges, but you can buy the bullets seperate from the cartridges. They would be heavy for the SKS and light for the M91/30, but no particular problems were apparent when I considered it. I do have some reloadable Winchester brass.
First thing I would do is slug the barrel. (A trip down youtube lane will show you the process.) Mosin's in particular are notorious for having groove diameters as wide as .312". Then, as Julio pointed out, get a reloading manual. This will tell you what bullets and powder recipes work. I'm looking at my Hornady manual right now: There are recipes for 130gr bullets with .308" diameter, 123gr bullets with .310" diameter, all the way up to 174gr bullets with .312" diameter.
What size bullets are you thinking about?