100 Straight makes a nice gage. It will reach 16" into bore. From them you can order it with 4 heads, I would get the 10, 12, 16, and 20 ga. I believe they were $450.00.
I have the Skeets gage, bought it from eBay for $15.00. Wasn't working, took it apart and someone put the ball in the bottom and the spring on top. Fixed it and had an issue with it creeping on the phenolic piece, called Skeet's and the worker on the other end said just to put a drop of crazy glue on the piece after placing the dial indictor down to the 1 (.100) on the gage. Works fine.
