Take a ride to the Lowe's and spend a few bucks on a roll of contractors paper. Its 36x36" and $12 will buy you about 150' of it... more than you'll ever need. You want to make sure you capture as much of your pattern as possible.

While you are there... get a 100' measuring tape... and make sure you are 40 yards away.... not crucial... but I've seen some pretty wild guesses in my day. I think your set up will be fine.... you just want to make sure that the gun is shooting Point of Aim to Point of Impact (read: straight).

Thanks Swamp. Never thought of the contractors paper. I have just been using butcher paper, but it doesn't catch the whole load at farther distances.

Thats the only glaring issue I've seen in what you are working on.... sounds like you are going about it just right. Shoot at the center and find the densest part of your pattern with your 10" circle template... hopefully its in the middle of your paper, covering your point of aim and configured about 60/40. (thats 60% of the pattern in the top half of the target, 40% below it.... 70/30 is ok too).

Looks ok by me @ 20 for sure, That pattern should kill birds @ 40 too, from what I can see on there, no big holes in it. To try either #6 or maybe a xtra full Undertaker shud close it up @ 40 though, I'd try the #6 first, maybe you know someone else you can bum a couple off to try? Had really good luck with wwsupreme buffered #6 and an undertaker in three different guns, so, I have faith in that choke,too. It's not a wallet buster either.
Less than hevishot or nitro, and definitely more effective than #7 anything, unless it's under 30 yds.

try using the winchester extened range
to be honest with you my moss. 500 would shoot great one season then the next it wouldnt pattern worth a crap so i bought a 835 and love it

Its all in how much you enjoy patterning...There are countless options setting up your turkey rig...An acceptable hunting pattern can be what the shooter deems all he or she needs for their range preference.. Lead had killed turkeys for years and will continue until non toxic shot regulations force it out of the game..for the most efficient patterning hevi shot, tungsten are above and beyond the capabilities of lead...With the Nitro, Hevi 13 or Fed HW pellets the performance can be made outstanding once you've matched a choke with a polished barrel and/or choke that compliments the load...For bare bones affordable performance with that 500 mossy and Hevi 13 6's the carlsons .660 as mentioned by another post is a great start..Shoot the Hevi13 3" 2oz 6's or 7's. Nitrocompany ammo would be your next step but the mentioned loads will be great..

Good luck on your setup!

I think you need to do some penetration test on hevi #7's. They are effective way beyond 30 yards.