Tim, it's just a personal observation, so don't think I'm being insistent. But, find you a Skeet's bore micrometer (there are other brands, new and used). They're not prohibitively expensive, and will turn out to be one of the most useful "gun accessories" you would ever buy. I have the ability to measure bores and chokes on everything from .410 through 10 bore, and have never regretted the purchases.