Re: AR15 cheap ammo?
Big question.. do you already reload? If so, and tiny groups are not a big deal, look at places like Midway USA (a sight sponsor) and buy bulk (1000pcs or more) of the Dogtown bullets or the Hornady 55gr FMJs. Fine for plinking. The more you buy at one time.. the cheaper it will be in the long run. Get 5000 primers, few thousand bullets or more, bulk surplus once-fired brass (1000) and couple 8lb'ers of Varget. Check out Powder Valley too.
If you don't reload currently, buy surplus or bulk Russian made stuff like Wolf,Brown Bear, Privi, etc. You can get 1000rds ready to go for about $220.. like $.22 a round.. which ain't bad for not putting any work/effort into it.