Your 1 in 20 twist barrel is the right twist for shooting sabot loads. Lyman has come out with a 12 gauge hollow based sabot slug that weighs 525 grains and it can be loaded in standard trap and skeet wads and star crimped over the slug. They do well in this twist rate and shoot flat for quite a ways. They are very accurate on riflef Hastings barrels and they hit very hard on the receiving end. The molds sell for $70.00 US over here and you would have to pay extra for shipping, but I think you would really like them in your gun.