The definition of sectional density is mass divided by cross sectional area (M/A).  As it applies to bullets it is the weight of the bullet in grains divided by 7000 times the diameter of the bullet (Cal) squared

M (grains)/7000 X (Cal)2

Sectional density alone in, general terms, predicts penetrating ability of the bullet but ignores many other characteristics of a given bullet that dramatically affect penetration.  One only has to do ballistic gel test penetration experiments on an expanding bullet to see that penetration is not as good at high impact velocities as at low impact velocities, since the bullet changes shape to a greater extent at high impact speeds.

Lead core hollow point jacketed or soft nose hunting bullets are complex in their behavior on impact.  As soon as the bullet hits, its sectional density starts to decrease as the nose expands and the lead core mushrooms and flakes apart due to frictional forces as it travels through the target’s tissues.  Lightly constructed bullets, meaning they have thin jackets, deform much more rapidly on impact than heavy jacketed bullets.  Also the hardness of the lead core greatly influences the rate of expansion with soft lead alloys deforming more rapidly than harder alloys.  The advent of partitioned lead core jacketed bullets was an attempt to limit the deformation of the bullet by stopping further expansion once deformation of the nose had reached the partition, usually located at roughly mid shank.  A useful second feature was the predictable retention of the shank portion of the jacket with the lead core.  Non-partitioned jacketed bullets suffer from jacket separations issues, especially the ones that are high BC bullets intended for long range shooting.  Bonding of the core and jacket has limited this problem, but it still can occur.

All copper bullets hunting bullets are much simpler in their internal structure in that the shank of the bullet is homogeneous.  No partition, jacket or bonding material to influence expansion behavior.  Copper, either as pure copper or gilding copper alloy, is much less ductile than lead and behaves more like a solid in its transit through a target than lead which behaves more like a dense highly viscous paste. Expansion of copper bullets is universally predicated on a hollow point which is intended to fill with fluid material on impact and be forced open by hydraulic pressure within the cavity.  Frictional forces then take over to peel back the copper in petals, like a flower.  The petals are themselves harder than lead and act like knives, cutting the tissues as the bullet spins through the target.  The rotational velocity, or angular momentum, of the bullet likely plays a key role in tissue disruption, likely not seen to as great an extent in the lead core bullet.  In fact bullet spin likely contributes to jacket separation from the core in lead core bullets.

The key to reliable expansion of a copper bullet is the structure of the hollow point and the behavior of the tip in the first inch of penetration.   The hollow point shape and depth greatly influence how readily expansion is initiated, and determine the range of impact velocities that will allow for reliable initial hollow point expansion.  So, a bullet that will expand reliably at a lower impact velocity, all else being equal, will have a greater effective range than one that needs a high impact velocity to expand.  When one combines a higher BC with reliable low speed expansion, the effective range of the bullet is greatly enhanced.  By our calculations based on BC measurements and minimum expansion velocities, the Bulldozer bullets have as much as 45% greater effective range than our leading competitors.  Also, because the bullets also deform less catastrophically than do lead core bullets, the sectional density decreases that occur as the bullet travels through the target are not as great so much deeper penetration is achievable.