You can't buy most of the select-fire weapons in Cataclysm without military/LE purchase orders or a machine gun dealer/manufacturer license. A good chunk of them have civilian semi-auto copies, though, and the older stuff have a few examples of transferables. (Though MAC-10s are pretty common, because the manufacturer registered a whole *bunch* of them immediately before the ban.) There are only something like 250K registered machine guns in the American registry, only half of which are transferable (which means you can own and use them). Some of those are inoperable (pre-68 deactivated war trophies, mechanical damage), some are unbuilt weapons kits with a valid registration, and some are basically just registered parts or lightning-links that aren't actually inserted into a host weapon - which would mean you'd have to find the weapon that it can be used with - a somewhat difficult task *now* and almost impossible short of sheer luck for some weapons in the zombie apocalypse.
In the US you don't have to have a safe or anything to have a firearm, even an NFA firearm (machine guns, shorty rifles and shotguns, suppressors, explosive/poison/incendiary munitions, 40mm 'flare' launchers) - some states and cities have their own rules, though. But it's still a good idea to have a safe - machine guns start around $3500 for SMGs and $7500 for assault rifles, and you do *not* want to explain to the ATF that you had a transferable machine gun stolen because you were a dumbass and couldn't be bothered to buy a proper safe for your very expensive toy.
There's a few ways to do this:
* We could arbitrarily declare that the machine gun registry freeze is dead by 205X, so now we have lots of civvie machine guns.
* We could remove the select-fire weapons from the gun store loot profile altogether, putting them into places like police stations and National Guard armories. (Police stations would have mostly submachine guns, National Guard armories having the select-fire rifles and LMGs and explosives)
* We could create new loot profiles for gun stores, splitting them into ones that focus on the sporting market, on handguns, and on tacticool stuff. Also, a few more profiles for gun store layouts, some having back rooms, others with longer target ranges and a smaller showroom. Also, pawn shops. Mix of sporting and handgun loot profiles and random other stuff, like used electronics and melee weapons. WHY DO WE NOT HAVE PAWN SHOPS.
Of course, getting in could be difficult without a key, and that damn alarm is a dinner bell for zombies should you trip it.
I'd personally recommend a mix of the three.
(Whew! Sorry, long post...)