Maybe my answer is simplistic and ignorant, but why don’t you just overbore your 28 gauge barrels to 24 gauge, leave the chambers alone and shoot it as an overbored 28 gauge. It seems that with modern plastic wads, the increased bore diameter shouldn’t really make that much of a difference. I have a 10 gauge that I shoot 12 gauge shells through using Gaugemates and it works great, the plastic wads don’t seem to be bothered by the increased bore diameter and seal just fine.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)