The bead at the end of the barrel and my eye for sights; whatever constriction Browning uses for Modified or whatever the choke that came with the gun is; and whatever turkey loads using 4,5, or 6 shot I grab out of the bin. I am getting less fussy about shells as I hunt in the woods and a long shot is 30 yards. A friend has killed her last 3 or 4 turkeys with her pheasant load of an ounce of 5s and the standard Full choke tube. As has been mentioned, bring the birds in close enough and much does not matter.
