Tried a few chokes and a bunch of shells out of my 1300 over the years, and I have settled on the Rhino .660 choke and Nitro Company's H378 plated tri-plex hevi-shot loads of 4-5-7. This combo has netted me a honest 316 pellets inside a 10" circle at 40 yards.
Now I see that a Hevi13 blend is now available from Cabela's spring catalog. Might try those shells when I run out of the Nitros.