Buy anything that has a detachable mag. Buy them all the time, with mags and ammo. Don't discriminate. AR, AK, FAL, PTR, CETME, HK91, SL8, Mini-14, PSL, M1A, Saiga 12, pistol caliber carbine....you get the idea.
That's the best advice.
And I agree, 18" is the magic number. My brain is off tonight. I seriously need some range time.