Paul-

Sectional density is related to bullet wieght and the diameter it acts through. This has a direct bearing on penetration and bullet effeciency.

As an example the penetration of a pencil would be different from that of a similarly fashioned depleted uranium rod when moving at the same velocity. You get whacked with the uranium one and at the some increasing impact velocity it will go THRU you while the Pencil penetrates minimally. That's the effect of SD.

Sectional Density in a properly contructed bullet means penetration and death if aimed appropriately. Momentum refers to the blow or impact shock delivered by bullets.The old 4 bores used an Ele had great momentum and caused great wounds but were poor killers due to lack of penetration.

SD-penetrates and kills
Momentum-whacks 'em and makes them know/feel they been horribly hit.

You want both--