A normal 12 gauge barrel measures about .724 thousands of an inch. By comparison, a factory full choke squeezes down the muzzle to about .700 of an inch. The normal way to get tighter patterns is to reduce the choke size even more. For example, many popular turkey guns come equipped with chokes that measure .665, and shoot turkey loads of No. 5 or 6 shot extremely tight.
National Wild Turkey Federation Special Events Coordinator Rhett Simmons, who runs the NWTF's annual Turkey Shoot and Outdoor Connection Inc., World Wild Turkey Still Target Championship—one of the biggest sporting clays and still-target competitions in the country—knows how important a proper choke can be. "Be careful not to throw too much choke on your gun. Too much constriction can create ragged patterns that can leave large voids between pellets. Voids in the pattern can lead to shots that miss the vitals."
A good place to start with a standard size 12-gauge barrel is with a .660 tube. Back-bored barrels usually do well with chokes that measure around .680 thousands of an inch. If you are interested in a particular choke, call the manufacturer or speak to your local gunsmith about their recommendations before making a purchase (NWTF's Turkey Shoppe at www.nwtf.org or 800-THE-NWTF offers a selection of choke tubes to fit most any shotgun). Then go out and use the choke tube with a variety of load sizes and even brands of shells. Odds are you will find one that makes a good combination for your gun.