What does sectional density mean? How does it effect bullet penetration?

Sectional density is a measure of a bullet's weight relative to its diameter.
A good way to see the effect is to throw a ping pong ball (low S.D.)as far as you can, then throw a golf ball (higher S.D.). They are about the same diameter but the effect of the difference in S.D. can be plainly seen.

The old theory was to use a bullet with higher sectional density in order to obtain more penetration.Bullet construction however is far more of a factor than sectional density so with todays new bullet designs sectional density is not an accurate way to determine penetration.As an example the 7mm 140gr failsafe or barnes x will easily out penetrate the 7mm 175gr bullets of conventional construction.

Well spoken Stubblejumper, and to mention the 284 cal. as well.

While the above replys are technically correct there's more to it than that.

It's not the case that just because a bullet has a higher sectional density of another that it will penetrate deeper. More specifically the correct statement is that "all other things being equal" this is "usually" true.

Examply: Hornady makes three 300 grain bullets for the .375 diameter guns....all with the sectional density of .305. One is a boattail spitzer shape, one is a round nose soft point and the last one is a round nose solid. Penetration of the solid certainly should be far superior to the other bullets and I won't speculate which would be better of the other two.

Just don't want to have someone camping on the sectional density as a magic number...it's just one piece of information in a sea of date to be considered.

It's why I really appreciate field reports posted by others.

S-jumper has pointed out a very important caveat for using SD as a measure of penetration. Bullet construction and impact velocity and, to lesser extent, caliber needs to be conditioned on. Comparing SD across bullets, even of the same design, can be a bit tricky as larger caliber bullet are generally constructed for considerably heavier game, again. For example, you can expect the 140 gr. 277 ballistic tip to be of considerably lighter construction than the 200 gr 338 ballistic tip; the latter will out penetrate the former despite the fact that it has a *lower* SD. One must also take into account how certain calibers (such as 338, for example) are dominated by magnums, and thus most bullets are designed a bit stouter to endure these typical velocities. The 358 bore is a bit the opposite; most commerical 35-bores operate at 35 whelen or so velocities; bullets are designed with this in mind.

The most vivid way to illustrate the importance of bullet construction is to compare an expanding bullet to a solid of the same weight and bore; the solid will, of course, penetration farther despite the identical SD. In fact, you'll find that FMC bullets of even a much lesser SD, caliber, and velocity will almost always out penetration even a stoutly designed expanding bullet.

As soon as the bullet hits the target; a whole new set of measures comes into play, and in fact, it is these measures that are relevant to killing power. Terminal (post hit) ballistics is where all the interesting stuff happens. The terminal ballistic coefficient and terminal SD are what we REALLY care about.

Not that you asked but another facet of SD relates to BC. For a given form and caliber, BC is directly proprotional to SD. Penetration is a function of RETAINED SD and velocity in the terminal phase of ballistics, which means the less it deforms and faster it is moving, the more it penetrates. This assumes of course that the bullet does not tumble.