It sort of depends on the choke size for a good answer. If it is a 555 definitely the hevi 7s, 560 -570 , perhaps the 6s or even the 7s again, 575 the Mag Blends and or 6s most likely, but this is not an exact science by a long shot.
As barrel length and choke diameter are not given it is harder to guess. Only the true test of putting the shells through the gun at a given yardage will tell for sure. The only other variables are the lot#, wind, humidity, elevation, temperature, shell components, .........etc.
Good luck with your recent purchase.
