Every thing I have ever read on this co-incides exatly with what Stan has posted. Have also seen the difference in number of shot per ounze for the two types, as I recal for #6 it was 218 for drop & 225 for chilled, or a gain of 7 pellets due to the very slight reduction of density.
Also in the early days one had to also name the tower shot was produced in when stating size as every tower had their own designations. An 1892 Lefever catalog for instance list a mumber of different towers with corresponding number per ounze, each being different. By their 1913 catalog they showed only one size which is essentially the same as today, only variation being from actual density of the alloy.
The Tower sizing which was finally standarized on was Tatham Bros of New York. With this standard the base line (Size B) was .170" with each whole size downward being .01" & half sizes being .005". I don't recall all the upward sizes bu BB is .180, which is .003" larger than .177" "Air Rifle" shot which we mostly call BB's. The development of shot is actually a rather interesting subject.
For any shot size larger than #10 simply put a 0 in front of the nubber & subtract from.170 & you get the dia of the shot as .170 - 06 =.110" or the average nominal dia of #6 shot. For #10 & smaller leave off the leading 0.
