15 year old, buying a Mosin Nagant?

nope, you need to be 18 to buy and own long guns, which Mosin Nagants are considered. also, Mosin Nagants are Russian, the Carcano is Italian, totally different rifles, only similarity is that they're both bolt action rifles, that's it. I think you should do more research and more experience before buying a gun, since you obviously know nothing about guns, period.