The Mossberg Ulit-Mag does not like really tight choke constrictions. Look into it more before you buy one but .685-.690 is about right. Try different brands of chokes like Rhino, Kicks, or Tru-Glo that in itself can make a huge difference. The gun likes the larger constrictions because of the way Mosseberg back-bores the Uti-Mag barrel.