/http://shotshell.drundel.com/pelletcount.htm
This chart is pretty useful. I didn't realize that there is only a 0.005" or 0.12 mm difference between #7.5 and #8.
Some years I am blessed to be able to shoot at a lot of doves. If I expect there to be more mourning doves, I reach for #8 and if whitewings then #7.5.
Although when I was low on shells last week I shot RST #6 copper plated out of my Trojan 16 and it dropped the birds DRT. Not the most economical load but it worked well on an assortment of mourning, whitewing, and Euroasian.