I have a Rem 11-87 with 21" barrel, I also have a Rhino .660. Nitros shoot in the 270's. You could save a little bit of money messing with chokes and spring for a box of 4X5X7 Nitros or their straight 7's and get some really good counts. Just my opinion. Just shot the gun 2 days ago with Winchester #6 Long Beards, the factory Remington Super Full shot in the low 200's consistently. The Rhino didn't like them and was putting them in the 150 range.
Ridge Rooster