Bill, I am now using the CSP Professional Bore Gauge.

http://www.brownells.com/.aspx/pid=6808/Product/CSP_PROFESSIONAL_SHOTGUN_BORE_MICROMETER_SET

My favorite gunsmith uses one of these so I had been keeping an eye out for a set. I found an older, complete 6-gauge set on eBay, cheap, from a retiring gunsmith. Wooden case, heads, setting rings for .410/28/20/16/12/10 gauges. I don't think I'd be willing to pay Brownell's price for that set, new, but used is a different story. It works and is more accurate than I am.