try a jellyhead choke and winchester extended range 3 or 3.5" shells and you will have a 40 yd gun. Have not seen that combo fail in a mossy yet!
Maybe you'd like to try mine....it's a mossburg too....has a modified barrel....#6s shoot great out of it:wink:I was also wondering how much barrel length would play into it as well. I've hunt with mudshark, a couple different cousins and a BIL it seems their shotguns make mine look like well.......me, short. (20" barrel)
Or a picture of blechk o'mama!:smack:I'd look at the Carlson's. I don't have that gun, but would guess in a .660; i'd go nonported. you can get them from midsouthshooterssupply.com for $17 bucks + shipping.
you can call carlson's and they will tell you what diameter they recommend. good guys there.
as far as patterning on fancy targets, go on a calm day and just take you some brown packaging paper - put it over a box or something (a poltical sign works well); draw you a dot and make a 10" ring around it - count your holes in most concentrated area, meaning you may have to adjust your point of aim if it's not directly concentrated - guns and different types of shells can all throw this off a little.