Bullets change shape during penetration due to the stress they encounter, and a bullet’s resistance to this stress equals its strength or toughness. A more appropriate explanation of the shape-shifting a bullet experiences during penetration might be “deforming,” which is defined in physics as, to alter the shape by pressure or stress. Of course it also means to open or spread out, and some bullets do just that. “Expanding” means to increase in volume or size. However, with expanding bullets, this is somewhat like saying that “there’s a 50 percent chance of rain” because with expanding bullets, “all things” are never equal. 375-caliber solids impact an animal or expansion medium at the same velocity, the heaviest bullet - the one with the highest SD - will most likely penetrate the deepest. With regard to non-deforming solids, there is some truth to this. A common cliché often used by gunwriters goes something like this: “All things being equal, the bullet with the highest sectional density will penetrate the deepest.” It also means that if two bullets of identical weight but of different caliber are compared, the smaller caliber bullet will have the higher sectional density. Simply stated, if bullets of the same caliber are compared, the heaviest bullet will have the highest sectional density. So, what is the connection between SD and BS? The formula for BS goes something like ½ Truth + ½ Fiction = BS. In short, BS is what you say to your buddy as you sit around the campfire, after listening to his story about the big one that got away or the 600-yard shot he made. The Wikipedia on-line encyclopedia defines “BS” as a common English expletive most often used as a description of incorrect, misleading or false language and statements. Sectional Density (SD) is the ratio of a bullet’s weight to the square of its diameter: SD equals a bullet’s weight in pounds, divided by the bullet’s diameter in inches, squared. A bullets toughness - resistance to deformation or fracture - has a plays a greater role in determing how much it will penatrate.

Sectional density does play some role in defining a bullet's penatration potential, it is not absolute.

if two bullets of identical weight but of different caliber are compared, the smaller caliber bullet will have the higher sectional density.

if bullets of the same caliber are compared, the heaviest bullet will have the highest sectional density.

Sectional Density (SD) is the ratio of a bullet’s weight to the square of its diameter.

Does Sectional Density Define Penatration Potential: Despite common wisdom, there are some major nuances to the idea sectional density predicts how deep a bullet penetrates. However, their construction and terminal performance vary greatly. 300 Winchester Magnum loads have bullets with identical sectional densities.

Higher calibre meaning