SBS recoil??

Hi - I have a 12 GA, 6-shot, Mossberg Maverick 88 Security model - I replaced the original stock with a pistol grip - it lives under my side under the bed in a length of 5-inch PVC pipe that I attached under the bed, perpendicular to the edge of the bed. I don't shoot it too often - the recoil can be painful after more than a few shots.

I'm intrigued with the idea of creating my own SBS - I'm familiar with the BATF process to do this. What I'm wondering about is how much more recoil I can expect with something shorter? I was thinking about a side-by-side or an over-under model - something where I can get the barrel cut down significantly and replace the grip with a pistol grip. I assume that these types of guns are all 12 Guage? If not, would the right loads in a 20 Guage or a 410 have enough stopping power in a HD situation?

I'd appreciate any ideas.

Thanks!