THE PROPORTIONA IS NOT A REPRESENTATION OF DENSITY!sergeante wrote:

The sectional density is a de facto measure of form factor.

No it does not. It takes weight & divides it by the cube of diameter (which would geometrically be a cube, not a sphere) to produce a RELATIVE proportional value of a shell's weight to diameter.sergeante wrote:

All your approach does is take the volume of an ideal projectile -- a sphere -- and compare it to the mass of an actual one.

YES IT IS. It does not take any real measure of a shells size & is NOT a representation of any real shape/dimension(s)/volume. It is a VERY SIMPLIFIED RELATIVE representation of RELATIVE size as a function of diameter.sergeante wrote:

It's not dimensionless at all, because the mass is a factor. Also, because you're using an ideal projectile, it's going to have the same volume for any given diameter, and proportionally the same difference in volume between one caliber and the next. That means your reference projectile could be anything of the same diameter, as long as it was a proportionally identical shape from caliber to caliber.

I am SIMPLIFYING things (the ONLY variables are diameter & weight), not complicating them.sergeante wrote:

This in fact is the whole point behind using sectional density in ballistics -- it's a direct measure of comparative mass for a given size. There's no need to complicate things further.