[mpipes]

I think it really depends on the origin of the group and its organization. Quite a few are going to be deserters, and they will likely have their issue weapon. Others are going to have gotten their hands on police/National Guard/military armories or sporting weapons. Most groups will have a variety of weapons, ranging from semi-auto/pump action shotguns to souvenir WWII weapons like MP-40s with little to any standardization. Military M-16s would be highly prized and coveted.
In my campaign, a large number of submachine guns had been distributed into the southwest US to fight the Mexican Army and significant numbers could be found in civilian gangs. These mostly consisted of Carl Gustav m/45s, Uzis, MAC 10/11s, Walther MPL/MPK (placed in production by MILGOV at FN plant in SC), and new built Stens (blueprints widely distributed by MILGOV). Moreover, large numbers of Mexican Army military weapons have found their way into marauder hands - MG-21 and G-3 clones for the most part as well as M-16A1s and a few FALs. Relatively few Soviet weapons would be encountered, but you could reasonably expect captured AKMs and AK-74s in the hands of marauders.
In other parts of the US where the Mexican invasion had not reached, it would be a motley mixture of whatever they could get. In my view, most marauders would be armed with short range weapons in popular calibers. A lot of 9mm pistols and carbines would be found. MP-5s from police especially prized along with M-16s. Ruger mini-14s. Lever action .38/.357 and .45LC rifles and carbines. 30-30 lever actions. 12 gauge pump and semi-autos shotguns. Relatively few scoped, high-power rifles. Significant numbers of AR-15s, as I think both CIVGOV and MILGOV would have issued them out to aligned militias trying to maintain order and many would have been promptly captured (e.g. ISIS in Iraq). A few well organized groups may be standardized (e.g. M-16/AR-15s and Glock 17 pistols), but those would be rare in my view. FALs, M1A1s, M-1 carbines, and the like would be found as well. Larger caliber weapons (7.62mm NATO, 30-06, 8mm, etc) would be rarer though.