michael-- That shotgun loaded up with Brenneke 3 Inch Magnum BLACK MAGIC slugs will be a hell of a performer. I am not sure if your shotgun is chambered for 3 inch, however if it is--you have a factory slug recommendation there. It is an ounce and as eighth of hardened lead and is a hell of a load.

If you can load your own BALLISTIC PRODUCTS has everything you need to make DANGEROUS GAME 12 gauge slugs that can match or maybe even exceed the BRENNEKE BLACKS.

I have a 23 inch barreled side by side 12 gauge smooth bore shotgun. OPEN CHOKE.

For those big massive slugs you do NOT want to have any choke. My side by side can shoot clover leafs at 50 yards with those brenneke slugs!

A hell of a kick though! I installed a mechanial recoil reducer and a Pachmayr High End Recoil Pad and that gun is NOW tame, though and I can shoot the big slugs all day. Before those two improvements the Big Slugs would knock very hard and I would guess your shorty 20 inch barreled over and under is VERY light and you will need to make some improvements like I did to make the Recoil of BIG SLUGS manageable in that shorty over and under. Good Luck!