you can get away without any difference between chamber and barrel with .410........ know this for fact as have made up redneck insert for .12ga with piece of pipe the right diameter set into a reemed out .12ga cartridge...the basic steel chamber inserts that are 3" long work and shells are fine to reload but pressure/velocity is low.... but looks like you know what your doing so will shut up and let you get on with it....
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