Sectional density is an important component of a bullet's ability to penetrate, and destroy the vital organs of a game animal requisite for a quick kill. Chuck Hawks clearly, concisely describes and compares sectional density here: http://www.chuckhawks.com/sd.htm so I'll not duplicate what Chuck has already set forth so well.

Dr. Martin Fackler has documented the better wounding of longer, heavier bullets. The FBI Quantico study has done the same, showing that longer bullets produce greater wounds and have greater lethality. The longer, heaver bullets naturally have greater sectional densities as well. Along with the better sectional density for a specific bullet design comes less velocity erosion with the automatically higher ballistic coefficient, and perhaps more important to the muzzleloading hunter, less wind drift.

It is not that bullets with poor sectional density have not harvested game; even the ballistically challenged round ball has taken its fair share. Under ideal circumstances, a broad side or honey hole shot, a bullet that expands rapidly usually works well, and sectional density is less important. When the angles are less than perfect and raking shots or through the chest shots are needed, muzzleloading bullets with poor sectional density have failed. By fail, I mean failed to drop the animal quickly, resulting in either a long tracking attempt or lost game.

I've witnessed too much of this, and a commonality has been bullets with poor sectional density. Sectional density is important in rifle bullets and handgun bullets, and muzzleloading rifles are in no way exempt. The field results from good (or poor) sectional density projectiles has been documented over the decades. And when the game gets larger and tougher, sectional density becomes even more vital.

A muzzleloading projectile with a section density below .200 is deficient for the more demanding shots on thin-skinned (CXP2) game, and for even a greater variety of shots on heavier (CXP3) game animals. I'll be specific as to what bullets look borderline, and of course you are the final judge of how, where, and why you might take a specific shot at an individual game animal.

• .40 caliber Hornady 200 grain SST: SD = .179
• .452 caliber Hornady 250 grain SST: SD = .175
• .490 caliber (".50 cal. Roundball") 177 grain = .105
• .50 caliber (.499) Powerbelt 245 grain: SD = .141
• .50 caliber (.499) Powerbelt 295 grain: SD = .169
• .50 caliber (.499) Powerbelt 348 grain: SD = .181
• .535 caliber (.54 cal. Roundball) 230 grain: SD = .115

• .458 Barnes Original 300 grain: SD = .204
• .451 Barnes Spitfire 285 grain: SD = .200
• .451 Barnes MZ-Expander 300 grain: SD = .211
• .451 Buffalo SSB 375 grain: SD = .263
• .452 Hornady XTP 300 grain: SD = .210
• .458 Hornady HP #4500 300 grain: SD = .204
• .50 caliber (.499) Powerbelt 405 grain: SD = .232

There are other considerations beyond sectional density that you might wish to evaluate. Pure lead pancakes at low velocities, inhibiting penetration. This makes the lighter Powerbelts, which are nothing but soft lead, and lead roundballs penetrate even less than their dismal sectional densities might indicate. The lead 375 grain Buffalo SSB is far more likely to get you where you need to be in less than ideal hunting circumstances.

Flipside, Barnes all-copper bullets by virtue of their 100% weight retention potential, have been proven to out-penetrate thin-jacket bullets by a large margin. So, though a 245 grain .451 Barnes Spitfire has a sectional density of .172, it penetrates like a substantially heavier bullet. The same is true for the new .193 SD Barnes .451 275 grain XPB, which has already proven itself by out-penetrating its 300 grain jacketed competition.

Using bullets with sufficient section density enable you to quickly kill and retrieve big game that otherwise would be lost in dense cover or with raking shots. It is something that Elmer Keith, Jack O'Connor, Charles Askins, the FBI-Quantico, Dr. Fackler, Ballistics Engineer Paul Von Rosenburg, Dr. Gary "Doc" White, and Duncan MacPherson have all universally embraced.