The reason I put the Warehouse link in is they came to the conclusion, for reasons they did not understand, that 21.75" was a magic barrel length that tended to produce best accuracy in many chamberings. I thought it might be something you'd want to try before cutting all the way down, just to see how it feels and shoots.
Yeah, a 300 yard indoor range is benchrester's dream come true.
It sounds to me like you may need a custom stock. When I was in my middle teens a number of standard commercial shotguns and rifle stocks would bruise my cheek, but I actually grew out of it. So it had something to do with my body proportions at the time. I don't know if it was arm length or neck length or shoulder position or what, but the fact it happened to me at any time in my life means it could happen to someone else, regardless of age.
It should be possible for recoil to tend to pull the stock away from the cheek rather than driving into it if the comb is properly shaped for the shooter. In your shoes I would probably start by putting an adjustable target type butt plate on one of the guns causing me trouble and see if either raising or lowering it above or below the original butt plate position would tend to correct the cheek impact issue. A custom stock maker who has a try-stock for shotguns should be able to find the right proportions for you, but those aren't so common these days.
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