By the way, this is the hottest anti-zombie ammo in 5.56mm on the civvie market:
DoubleTap Ammunition
It's a ballistic twin to the US SOCOM Mk262 Mod 1 round. This is the stuff that was fielded by SOCOM after all the stopping power issues with M855 ammo in combat operations. It is very nasty fragmentation type stuff. Stability issues may manifest themselves in 1:9 twist barrels, so make sure you test it in your barrel, don't just assume it will work. It does work fine in my 1:9 barrel and shoots perfectly respectable sub .6" groups at 25m using the close range iron sight aperature from my 1:9 16" carbine (even at temps in the high 40's F.) That is way more than enough accuracy for any self defense scenario. It would shoot even tighter groups with optics or the long range iron sight aperature.
Double Tap makes HOT ammo in a wide variety of calibers.
Federal American Eagle XM193 is also very good, will work in any barrel, is about as good a close range armor penetrator as you'll find, and violently fragments 85% of the time.
I have never had any problems with any
good condition MILSPEC GI type mags in an M-16A2 or newer model of AR.