I have seen 55gr FMJ from Wal-Mart shoot 6" groups at 100yds with a scoped rifle and then the same rifle print a sub-MOA group with 69gr SMKs.
WWB (winchester white box) seems to shoot better than most surplus or cheap ammo. I don't know who said "junk in =junk out" but it fits.
Not even a match grade barrel on a custom rifle will help some junk ammo.
Most production grade ARs probably shoot 1-2MOA with decent ammo, better triggers help.