If we're talking economy then 7.62x39, hands down. Last year I bought several crates of surplus ammo for 18 cents/round, I've seen .22lr sell for that lately. And this week I was hitting some local big box stores & gun shops for ammo and although there was no .223/5.56 to be found, one shop had several crates of Brown Bear 7.62x39 for sale (no limit). And the pre-panic price for most new production steel (Wolf, Tula, etc) is 25 cents/round. Best price I've seen .300 is Remington for 60 cents/round, still cheap compared to many other calibers but not 'reloaded 9mm in bulk' cheap. The biggest weaknesses in the 7.62x39 AR are the tapered case in a straight AR magwell, ASC mags are the best solution for this, and there are AR47 style lowers that take AK mags. And the bolt face, which is thinner than a 5.56/.300 blackout. I've not had any issues with mine in close to 2k rounds but I did recently upgrade to an LMT enhanced bolt as insurance. Even if I broke a bolt every 1k rounds that's still cheap shooting. And I'm not picking up spent cases. But if you prefer .300 don't let reloading scare you, it's simple once you know what to do and starter kits with single stage presses can be had for $100.