In my opinion the best bang for the buck today is still the orignal USGI 5.56mm M193 ball ammo. You can buy it from any number of sources in it comerical box labled as Winchester White Box Q3131 (US made) or Q3131a (Isralie made).
The penetration of the M193 55 grain ball round is better than the M885 62 grain inside 300 yards. The M885 does not start to out perform M193 untill out past 400 yards.
M193 is very inexpensive, $3.50 a box of 20. Reasonably accurate, and a very effictive man stopper out to approx 500 yards in a 20 inch barrel.
Just my .02