Your gun has not been altered. I have a 21 20 that I bought for my grandson 20 years ago that was unfired. It is M&F and mikes .008 and .020. I bought another for a second grandson and it is .014 and .020. Winchester mod. is normally pretty open. I had a 12 made in mod and full and it was .015 and .030. My 20 that I had made in 1967 has skeet bores of .000 and .008 and mod. and full of .020 and .030. The 410 barrels IC and MOD are .004 and .008. All shoot great.

AWC