I bought an a400 26 inch barrel 2 years ago and I shoot federal tss #9 and Longbeards 6s. I have the Carlson.660 Longbeard choke and a Carlson.680 choke. I tried the federal 7s and they were good too but the 9s were the best. Both of these chokes are great in my opinion but I'm hunting with the .680. I like a little more forgiving pattern cause I've had a tendency to miss. With the 9s and the .680 it's a solid 65 yard gun and the .660 will add about 10 yards. I initially went with the .660 and I missed a bird opening weekend last year and I added a ff2 with the Burris speed bead after that. I hope this helps!
Proverbs 3:5-6