Bushmaster
DPMS
Armalite
I believe Ruger makes a good one now as well.
I’ve had the Ruger 556 since July...er, uh...then the houseboat sank.
But up ‘til then, I had so much fun with it, switched from the AimPoint red dot to a Leupold so I could see better at 100 yds.
Darn house boat...!