The most easily attainable ammunition in this country (USA) for an assault type weapon is"
5.56mm
I had the same issue and chose the Arsenal SLR106FR (very high quality)
I like the 7.62x39 and the 5.45x39 ..but I already owned two AR's so finding, buying, stocking another caliber seemed....out of the question.
The mags for the Arsenal also fit my existing web gear
I buy, shoot inexpensive Wolf 55 gr or 62 gr (FMJ and HP) and the Arsenal goobles it down just as easily as the more expensive 5,56 or .223
Look at the dozen or so companies that make .223 or 5.56mm ammo in the US.. you can easily, readily get plinking, hunting/defensive ammo, SS109, or match grade target rounds
(I practice/plink with Wolf 55gr or 62 gr but have a mag full of Hornady 75 TAP ammo for the house)
Domestic ammo supplies are not usually hit as hard as imported ammo...all the inexpensive ammo is "imported"
Imports can be delayed, stopped..(executive orders)
Go with 5,56