While realizing it will vary some according to brand, granulation & individual lot of the powder as well as size & alloy (hardness) of the shot an approximate equal volume is found at 2 drams-1oz. For other shot wts simply mltiply the wt by 2 to get the drams of the equal volume load. In looking over several old combo shot & powder dippers I often find them marked in dram & 1/8oz increments.
This means that if you double the size of the cavity to go from one oz up to a whopping 2oz for the big 8ga, the powder should also be doubled to 5 drams. The dipper however will only read it to 4 drams for it has those 8 graduations marked in dram steps. On the ones I have actually measured I have found the shot wts to be pretty close to what they say. I have always used the equal volume method for the powder charge so just set the dipper to the desired shot wt.
I do have one old dipper incidently which goes up to that 2oz of shot. Industry standard set the volume for a dram of BP as .115 CuIn many years ago, shot goes about .288 CuIn per oz. .115 x 2.5 = .2875 so the 2-1 combo is sbout as close as you can get it without weighing out individual loads which is fine if one has nothing better to do. The volume measure is quite sufficient for my use. The bird doesn't really know, or care, if that 2 dram load contained an exact 68.359 grains or if it was just somewhere between 62 & 75.
