Congrats - that's a great price for the 20. Post pics when you pattern it. Bet it would shoot well with TSS straight 8.5 or 9s.
I have the same gun but in 12 gauge. The 12 was factory threaded for Win chokes. Had SumToy drill/tap it for fiber optic sights couple years back and patterns Hevi-13 straight #7s nicely through a .660.