5.56mm is fine for 600meters as long as you have the right combo of spoon and peas.
I have some of the peas for that range (77gr Nosler OTM), but not the spoon.
Ironically the long range 77gr bullets are the best for close range defense issues as well.
Very expensive ammo though.
This is me on the 400 meter range back in the day. I'm the one on the end with the M203. I was actually shooting right handed that day because the brass from my A1 kept hitting me in the face.