The sectional density (SD) of a bullet would sure change upon impact should the bullet lose much weight thru fragmentation & expand greatly.

SD = bullet weight/7000 /(diameter^2), as the diameter increases the SD decreases, as bullet weight decreases (fragmentation) the SD decreases.

But same caliber bullets having same weights but different shapes have same sectional densities (SD), but not always equal BC's & FF's.

Bullets having equal BC's may not have equal drag resistance - FF values

From the ballistic experts at Berger:

"The problem with BC’s is that they combine the effects of mass and drag into one number. So if a bullet has a

*high BC*, you don’t know if it’s a medium weight bullet with very low drag, or a heavy bullet with high drag.

**The reason this is important is because if a bullet has a high BC just because it’s heavy, it will suffer from having a depressed muzzle velocity, and performance will not be as good as the high BC implies." - **quote from Berger

https://bergerbullets.com/form-factors-a-useful-analysis-tool/ BC=SD/FF, BC = ballistic coefficient, weight included in SD. Many zippy & pointy light bullets having modest BC values are good long range performers. Shooters get hung up on BC values thinking that they are the only measure of a bullet's capability to overcome drag.

The extended Berger table shows green, yellow & red boxes. Are your bullets in the green boxes? Would any long range shooter seeking optimum bullet performance neglect SD as a factor in reducing drag? SD has additional importance than evaluating the effect of a bullet hitting meat & bone. The folks at Berger did apologize for the math used to describe this stuff,

The Hornady 4DOF ballistic calculator does not use G7 BC values, apparently Doppler radar tracks bullet velocity and individual drop & wind values may be extrapolated from these Doppler radar data. No G7 drag models. Various other APs don't include SD values and rely on provided BC values. Bullet shape remains constant (except for those melting meplats).

A .45 hand gun having a double stack magazine would have a grip like a 2X4. - fatter bullets.