Since no one else mentioned it... Make sure that your upper on an AR actually is a 5.56mm one and doesn't have a commercial .223 chamber or some sort of funky out of spec chamber if firing 5.56mm ammunition. If you have a .223 chamber, don't fire 5.56mm ammunition in your rifle. Usually nothing bad happens, but every now and again, something does.
For quite some time, before I ended up with two 1:7 twist firearms, I'd keep the magazine in the rifle of my 1:9 carbines and 1:12 rifle loaded with relatively light Federal JHP ammunition. It might under penetrate, but the gelatin tests I'd seen when I'd bothered to look it up suggested that it might make a mess.
Later, I just filled most of my mags with Prvi's M193 type fmj, which I ended up with a lot of. I still keep some magazines loaded in one place (plus a 1000rds on strippers in a can) of "real" M855 green tip. ( I don't have access to M955 or I'd keep some handy.) Little better range, and a little better penetration under certain circumstances with the green tip stuff, esp since I no longer have my old SP1 to worry about. Some people keep it as their primary loading, but I don't have enough of it and it's expensive lately.
If I could afford to stockpile it, I'd probably use Hornady TAP simply because it tends to be more consistent than bulk pack or government contract FMJ.