Good explanation, Jack. You're correct on the Skeets. The interesting thing is that a set with two heads is the same price as two separate complete units. Might as well buy both units. Gives you an extra tube and dial gauge, just in case. You can still swap heads.

Gnomon, you must have a REALLY overbored Merkel there. My 20ga Sauer approaches .630, and my Skeets works fine.