I have both, the mcgowan in 6x45 and the ER in .223, BOTH shoot nice sub moa at 100 yds and about 5.5" @ 700 yds. Borescoped both when new and the mcgowan was sharper/smoother. The ER was sharp, smooth but had a tad of roughness in the grooves, but they both shoot very sweet.
BOTH clean up very easy and do not mine copper at all.