pcrowe said:Wooger,
I shoot a 6mm turbo 40 from Robert Whitley (6mm AR. com). I have two uppers and am on my third barrel now. They are very accurate and reliable. Good barrel life too. If you want something with better performance than 223, this is the ticket. For 600, a 107MK at about 2850 shoots very well.
Good luck,
Phillip