Some variables need to be established in order to come up with a sensible answer.
The first question is geography. In some parts of the United States, local governments have imposed various restrictions on ownership of firearms. Other localities might favour shotguns, or large calibre hunting rifles (for deer, moose etc.), AR-15's (shooting small game or "varmits") or handguns (urban areas). So characters are not going to go around like kids in a gun store and just pick up what they need - they might only find weapons which are of limited utility for them, even if the former owners were satisfied.
The second issue is ammunition. Ammunition needs to be carefully stored otherwise it will degrade. Ammunition sealed in metal ammunition cases is probably ideal, but factory packaging that resists moisture is second best. Ammunition stored in relatively temperature controlled environments will also last a long time. Otherwise, the primers and propellant will begin to deteriorate, and the casings can also corrode (especially things like Russian steel cased ammunition - lots of surplus Russian ammunition was sold starting in the 1990's to go with things like SKS rifles).
The best way to store and carry ammunition
The third issue is just what sorts of weapons are needed. "Monsters" are probably not going to be affected by .22 Long rifle ammunition, but keeping your game pot full might depend on your skills with it. Human predators may be deterred by a .22, but you will really need something more powerful to ensure the target stays down when hit (.38 for handguns is about the minimum. .223/5.56 X 45 has become almost ubiquitous in the US for AR and Mini-14 style weapons, but .308/7.62 X 51 is a far more reliable choice. Larger calibres are also needed for larger game, or to reliably engage at long ranges.
Good for big game, not so much for rabbits
Movies and political myths aside, fully automatic weapons are not available to civilians in the US, except under very tightly controlled circumstances (essentially the trade for the very limited number of "grandfathered" weapons which are now collectables). Military armouries and police stations will be where these can be found, but over the decades looters will have been using bulldozers to knock down the vaults and without proper training and fire discipline, the ammunition may have been blown away in blazes of glory. Carrying a belt fed machine gun is a pain due to the size and bulk, and even a proper automatic rifle like an M-4 will require a load carrying vest capable of holding 10 magazines if you are serious about using automatic fire.
There's a reason they dress like that
When you get down to it, any competent machinist can make a firearm with hand tools (the Sten gun from WWII was designed to be built that way, and Pakistani gun smiths can make replicas of virtually any firearm you can name). Hand loading ammunition is possible so long as you can collect the casings and have access to the tools and chemicals, so you are not just limited to scavanging. In a really post apocalyptic environment, it may even be more sensible to make or find a black powder firearm, as your ammunition resupply will be easier to solve (you can make your own gunpowder and even your own shot or ball projectiles).
The AK will be ready next week
Your ammunition will be done shortly. You said 15 cases, right?
The other avenue of approach is to relearn the art of making bows, especially longbows (warbows with 100 lb draw weights) or steel crossbows (using a spanning mechanism, you can have up to 1200 lbs draw weight). These bypass issues like ammunition (although you still have to find or make arrows or quarrels), and are much quieter than firearms as well. Bodkin pointed arrows or quarrels could penetrate mail armour, so should have some effect on monsters, unless you are fighting Godzilla, in which case even a Barrett light .50 rifle is pointless.
Making a longbow is an art
At 1200lbs draw, things will be uncomfortable for the target