I'm new to shotguns and shooting shotguns. I bought a 20 gauge tri star to start and shoot full and I M chokes for trap. What modifications are out there that would make my 20 similar to a 12 gauge when I'm trap shooting? I'm scared of the 12 gauge recoil. My loads are 2-3/4 inch 7/8 oz 8 shot. Velocity 1200.

Physics being what they are, the recoil from a 12ga aren't significantly more than a 20. 20s tend to be lighter, and shoot 1/8 oz less shot. Drop to #9 shot, most trap ranges don't allow 7 1/2s, find a skeet tube and shoot skeet and mod.

7-1/2's are standard trap loads. 8's will be fine from 16 yd line.

Skeet either 8's or 9's, 9's being pretty much standard in skeet. I've seen some skeet ranges limit shot to nothing larger than 8's.

The only 'modifications' would be to make sure your stock is set up correctly for you.  I don't know what adjustments are available with the tri-star, but some semi-autos have various shims that allow you to adjust cast (side to side) and drop (vertical) of the butt stock.  Find someone at the club that knows how and what to do.

So if I can handle the recoil from a 20 with a full and improved modified choke I should be able to handle a 12 gauge?