Going out with the Remington 30.06 every once in awhile probably needs 150 rds or so in storage. That probably would get expensive stocking up.
The FUN stuff, full-auto or not, would be the .223, 7.62x39 and the 7.62x54. Rifles can be cheap and ammo is even cheaper.
7.62x51 is also often available pretty cheaply as foreign military surplus, made to NATO spec though. Indian, Venezuelan (or was pre Chavez at least), Portuguese (which is mostly what I've got for my mostly Portuguese G-3) and even Brit (pre War anyway).