Say you have two barrels of water. One barrel is 10" in diameter; the other is 20" and for convenience and to match the SD formula to get the area of each base we square them, so the 10" base has 100 sq in, the 20" has 400 sq in. So just to keep it simple, lets say water has a weight of 1 ounce per cu in and that we desire the magical "sectional density" of .250. We solve for the weight. Since SD= W/A then SDxA=W; .250x100= 25; or .250x400= 100. The water in both barrels will have an SD of .250. The 10" barrel will have 25 cu in of water and the 20" barrel will have 100 cu in of water. But since the surface area of the 20" barrel is 4 times larger than the 10" barrel, it needs 4 times the amount of water to have the same SD and therefore the same depth of water.
SD shows only the amount of material ahead of a given amount of surface area. Various materials have different densities and so will have different lengths for same diameters, but within materials of similar densities, bullets of same SDs will have same average lengths. Naturally a flat point will be shorter overall than a spitzer, but their average length will be the same. Bullet length is only indirectly part of the penetration function.
If thought is given to penetration it is concluded that the primary positive force of penetration is momentum, shown as Mass x Velocity. The negative forces are impediments to the bullets path and the Ballistic Coefficient (not to be confused with in flight BC) of the bullet. The terminal BC changes rapidly as the bullet contacts the impediments. varmint bullets turn into virtual shotshells with dozens of small pieces of bullet, each of which has a very small BC and gives up its velocity very quickly. the other extreme is the solid which maintains its shape and changes very little in the interactions with the impediments. Big game bullets fall in between the two designs. The bullet is pointed in order to slice through the air with minimal loss of velocity, then on impact the nose does two things. It flattens out so that it will meet the impediment with a surface that pushes rather than slices, and the diameter of the frontal area increases which creates a larger surface area to push through the impediment. Since this pushing requires more energy (momentum) than did slicing, the bullet transfers more of its energy to the impediment. the transfer results in an energy wave that stretches soft tissue beyond it's limits and the tissue is torn, so after the passing of the wave the tissue snaps back in place but is no longer functional because it is ripped apart.
So an effective big game bullet will be one that has a flat surface area as it passes through the animals tissue and sufficient diameter either before expansion or after if expansion is quick enough to be relevant. the surface shape and diameter is then combined with sufficient mass and velocity for the necessary momentum. Momentum is a little deceiving since two bullets with the same momentum will have entirely different reactions to impediments. A.224" 50 grain bullet can be driven fast enough to have the same momentum as a .308" 180 grain bullet, but the high velocity is shed at a much faster rate than the slower bullet, so massive shallow wounds result rather than less intense deeper wounds, But that said, penetration is a function of a bullets internal BC, it's retained mass, frontal area and its momentum. SD gives a quick shorthand indicator of potential penetration, but that is all that it is.
Just kidding, but a hunt report with plenty of pictures would be great! I am sure it would be a unique an exciting read. Lever guns and Buff sounds exciting.
