The most common and easy to trade for/with Military, DHS, and local, county, state, and federal law enforcement is 40 cal for pistol, 12 ga for shotguns and 5.56 for rifle. If foreign troops are used, (NATO and UN) then 9mm, 7.62x39 and 12 Ga. Trading with your neighbor? 22 cal and any size shotgun bird shot will be around at first in quantity.
ok this is just a suggestion of course but you need to keep it simple sily (KISS). the only thing one person needs is a long rifle(for the long shots), an AR to be able to put rounds down range fast(throw an optic on it and it will give u some range), a shot gun for up close(some buck and bird shot, u wanna have bothe trust me), and 2 pistols (same caliber). the more different types of weapons you have the more diffferent types of rounds you have to buy. 5.56, 12g, .40 or .308 and it depends on the long rifle u get. i do not recommend a 9mm even tho it has its advantages. a .40 or .308 works better trust me.