Another fly in this admittedly interesting jar of ointment: you can't go by "book" figures to obtain accurate pellet count in a given load. Before you base your assumptions on 350 7 1/2's per ounce, you need to check each individual brand of shell (even from the same shotshell maker) to see how many pellets you have. The "book value", I've found, is just a rough average. I've found 6's--talking supposedly American 6 here, not the smaller British 6--that count out around 200/oz; others close to 250. On either end, that's a 10% deviation from the "book" number of 225. And really bad news if you're comparing the two extremes. Likewise, I recently counted Rem Gun Club and STS 20ga 7/8 oz 8's and found a significant difference there as well.
Obviously, if you're going to compute density accurately, you can't just assume that the load in question contains the number of pellets it should contain based on "book" figures.
