Just took one mag to the range today.
I put 18 in a 20 round colt mag, no failures of any kind.
The other mag was a 30 rounder filled with WWB 223.
shooting was all done at 100ft.
Offhand with a tripower I was shooting high and right with both ammos.
Left group is 30 rounds of WWB223 offhand no rest, shot a little high.
Middle group I switched mags to the guat556 also off hand, POI was a little higher than the 223.
To make sure that the Guat ammo was shooting higher and it wasn't just me getting worse, I sat down with a benchrest bag, popped on the aimpoint magnifier and aimed at the X on the right. CLEARLY the guat556 shot higher.
Later benchrest, magnified groups of WWB223 were dead on, but not pictured.
So the guat is clearly faster, but pretty darn accurate itself. THat one on teh right was a pretty decent group for me.