Hey Lutes,
I've never used anything but the Lyman 17A's, so I have no experience with anything of a larger ID. I have two others on my Old Renegade; one for the roundball barrel and one for the fast twist LRH barrel. Overall, I have no complaints.
If anything, I notice that I see quite a bit of barrel when I have the Merit disc opened up a bit. Even at it's tightest setting I can still see a bit of barrel. As my eyes got older, I had to go to a wider post. When I did that, my groups actually tightened up.
galudwig