I own to Mosins. I think they're absolutely terrific. However, if you have to use one for home defense, you'd better think of it as a spear and use it with bayonet fixed. Because of its great weight, really low rate of fire, and probable difficulty of being able to fire and work the bolt smoothly in a CQB situation, I really think of it as a spear that can also shoot, when I imagine situations like that.
The other thing is that the cartridge is enormous and powerful. If you fire it indoors without hearing protection I don't know what kind of damage you'll do to your hearing. The round will certainly over penetrate. As awesome as it may be to speculate about the kind of damage that a 7.62x54 JSP will do to a crackhead, I can't think of any socially responsible way to fire that weapon except for pointed downwards down a staircase as the crackhead charges usp.
When I think of that weapon in the context of defending your house, I think of it as a tool for someone who deep down inside really wants to kill a man using hand to hand combat, but who wants the ability to shoot to be there just in case. In other words you'd have to be deranged on some level to seriously consider it in that light.
This thread is full of win! I enjoy shooting mine, but it would be one of the last guns I would use for home defense, unless I wanted to beat someone to death, and even then, it is too big for my hallways.
AND: bullpup! http://www.cbrps.com/Products.html
I like SHTF gear to be the most common stuff around for ease of finding ammo/parts. 1911s, Glocks, AR15, REM 870. That being said the MG is an inexpensive, hard hitting , dependable firearm. I just wouldn't have it as my only gun. SKS is another good low cost zombie weapon.
I have to go with Roaming East, all the others who've made the points about "define SHTF" and "will you be able to get new ammo, or need to use your own and what you can find" - all that. I'd go with something like an M-16 derivative.
Or, with that op's current stock, I'd recommend a good handgun as the next purchase. Sounds like he has short range home defense (12 ga), varmint/light hunting (.22) and long range makes-big-holes (M-N) covered, personally I think a pistol or two (again, in an easy-to-find caliber) would be a good next tool.
But I'm just another guy spouting off on bullshido, so whatever!