I bought one years ago on eBay for $15.00. It was in a homemade wooden case and looked good. Trouble is, someone took it apart and put it back wrong, the had the spring on the wrong side. I fiddled with it and got it to work and it will give you re-peatable readings.
I called them and wanted a ten gage head, the fellow told me to adjust the phenolic tube farther down to allow the balls to travel farther. It still isn't enough for .775 or larger for earlier 10 ga bores.

For the money they are good, if I had the money I would get the 100 Straight with the 4 heads for $500.00

David