IF by "Tube profile" you're referring to the left barrel near the end of the chamber (as seen from below), that looks more like a bulge than anything else, at least until one looks at the side view and sees it's the groove. I think that octagon-to-groove looks like a prescription for a stress riser, which is probably why it's so uncommon to see.

Last edited by Dave in Maine; 10/04/12 06:38 AM.
fiery, dependable, occasionally transcendent