A two diamster bullet is caliber size at the base and bore size towards the ogive, most are divided at a cannelure. How many diameters I don't know, but two if you omit the taper.
This. Two different diameters in the straight shank of the bullet; it has nothing to do with the bullet nose taper or a boat tail. The straight section is stepped down at some point, with the front section smaller diameter than the rear. It's essentially the same idea as a bore riding cast bullet design that employs a long straight nose section at bore diameter (for example .300" for a .308" bullet) that rides on the lands of the rifling.
If a guy "can't figure it out", one can always ask or go look it up.
