My vote is for the Grendel.
I am probably biased as I have 4. I have a 20” M4 that groups 2.5” or less at 400 yards consistently with Hornady black. A ruger American predator that shoots .25 MOA with the same factory ammunition. I have just had such good fortune with that cartridge, so I don’t deviate. There is a huge difference when the bullet impacts steel at 400,500,600 yards over the 223. It’s just an efficient, flat shooting and much harder hitting cartridge in an m4 platform. That’s what I would use.
