It's called an Obrez (which I think just means 'short' in russian). To make one in the US you'd use the SBR registration process - I believe Form 1 to DIY or Form 4 to buy it from someone else. Not a FFL Dealer, so please consult one before considering. You'd have to leave the donor Mosin intact until getting your approved paperwork, IIRC.
Having said that, looks fun. Kinda like the 3-round, .30-30 revolver I once saw in a collection.