.555 at muzzle....hmm. I don't this rifle will be to accurate. Somehow my experience is, that every time I turn the barrel below .625 dia at muzzle ( for 7mm or .30cal ), the barrel shoots less accurate than its potencial. Apparently thinn machined muzzle expands dia of barrel bore to some extent. Bill Calfee, the gunsmith and writer from Precision Shooting has couple theories about it. It is very important for the accuracy of the barrel, that dia at muzzle be touch tighter than dia at the start of the chamber. Exesive muzzle thinning destroys that dependence.