I recall GJM commenting that the 20 gauge slug penetrates better than a 12 gauge. Hopefully, the man himself will comment here.
Three specifics: Is the 3 inch loading required to achieve the superior performance? Given that a typical 20 is a bit lighter than the equivalent 12, just how bad might the recoil level get? Any particular brand/type/weight of shell to be recommended?
Insights and prior research appreciated.