I'm surprised nobody suggested simply using a digital vernier caliper to measure the diameter of the rims of the ammunition you are attempting to use, and to also measure the diameter of the rim recesses of your barrels. You mentioned measuring with a micrometer, but that would not be the measuring instrument to use to measure the diameter of a rim recess. The caliper would also show you if your rim recess is "out of round", which seems unlikely unless the gun was used as a club to finish off a wounded game animal. I have two L.C. Smith Ideal Grade guns in 12 and 20 gauge, and never had any issues with use of factory ammunition in either.

Here's a link to a chart with shotgun chamber dimensions.

http://www.dave-cushman.net/shot/shotshellloads.html

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug