5.56 Doesn't Shoot As Well As .223 Remington Format
I have a Mini-14 Ranch Rifle, Mod 5801, 1:9 RH twist. This rifle hates to shoot the 5.56 NATO spec rounds. You couldn't cover the groups with your whole hand spread out. However, in the .223 Remington spec, it loves the Winchester Ballistic tip 55gr, Winchester Varmint HV 45gr, Federal Gold Medal 55gr, and Fiocchi 55gr FMJ. Best groups in mine were with Winchester Ballistic 55gr, with a 1 inch group of five shots.
Of course your experience may differ.