One rarely seen shot size, at least in the U.S., is #7--which is the same as the British #6, apparently the most popular over there for shooting driven pheasants. The nominal diameter is .10, and there are approximately 300 per ounce, as compared to 350 for 7-1/2's and 225 for 6's. That 350/225 gap is the biggest in popular American shot sizes, and 7's fill it neatly.
Bought some high antimony 7's from Ballistic Products a number of years ago when working on handloads for the 28-gauge, and was very impressed. A load of 7/8 ounce in the 28 killed wild roosters well out to 40 yards, using a typical modified choke, and when processing the birds for the freezer found they definitely penetrated better than 7-1/2's.
Have always suspected 1 or 1-1/8 ounces of hard 7's would make a great pheasant load in the 16, just as it does in light 12's in England, but so far haven't tried them in any of our 16's, apparently because of having too much factory ammo on hand. :-)
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A buddy is a firm believer in that load of 7s for pheasants in a 12 ga and talked me into buying a couple of bags years back. I think it is the perfect shot size for the vast majority of grouse, both woodland and plains, and in the smaller bores it would be a better choice for anything one would use 6 shot but I don't see myself switching to it any time soon.
