Judging from the brass left at the range and from what is available at the
small nearby drugstore, here is what is most commonly available for ammo in order of quantity found.
If you reload, this is a good list for what is available for free pickups
too. (of course not the rimfires)
22 long rifle
12 gauge shotshell
9mm pistol
30-06
223
38 and 357 pistol
30-30
308
various 40 and up caliber pistol
There used to be lots of 7.62x39, (though non reloadable)but no more, don't know why.
Get a bolt action 30-06 with scope and learn to reload both jacketed
and cast bullets. Then you can make your own bullets if needed and also
load down to quieter, lighter loads when needed. There is load data for
most any powder made from bullseye pistol powder to the slowest
magnum powders. And 308 caliber has the largest selection of bullet
types.