What does this mean.....?

[Harry]

In reading the Lyman 49th ed. reloading handbook. I don't know what a couple of terms mean in the load data sections. Directly below the bullet figure are the letters BC and SD with decimal values. I cannot find any glossary that lists these. I thought maybe SD stood for standard deviation, but what is BC, bolt circle? Excuse my ignorance. :confused:

Harry

[oneokie]

BC is ballistic coefficient.
SD is sectional density.

[BCB]

Sectional Density…

Ballistic Coefficient…

[462]

Harry,

Ballistic coefficient and sectional density, most likely. The meanings should be explained in the book.

[BeeMan]

Don't have that particular manual handy but SD and BC stand for sectional density and ballistic coeffiecient, respectively.

SD is an indicator of how much mass exists per cross section. Think of a pencil compared to an equal length of steel rod at the same diameter. The steel rod has the higher SD. Similarly, a long steel rod has a higher SD than a shorter section of the same diameter rod.

BC introduces the effect of form. Think pointed pencil vs. a pencil that is not sharpened. The pointed pencil is more efficient in flight.

[Harry]

Thanks everyone for the descriptions. I'll keep looking in the Handbook. I haven't read every page yet, but I've read quite alot of it. Boy, was I way off. Being a newbie, I never heard of either of those terms.:-D

[AZ-Stew]

BeeMan covered it pretty well, but I'll add a pinch more.

Ballistic Coefficient is an indication of the rate at which the bullet will slow due to air resistance in flight. The higher the number, the longer it takes a bullet to slow, or said an other way, the higher the number, the longer it will retain velocity. This translates to flatter trajectory, since the time of flight is reduced. The shorter the time of flight, the less drop.

BC is also an indication of how well the bullet will deal with cross winds. Again, the higher the number, the less affected it is by the wind.

Sectional Density is a factor in the bullet's ability to penetrate.

Regards,

Stew

[runfiverun]

the closer to the number 1 the better abything over .5 is pretty outstanding
b.c is kinda a boolits ability to maintain it's momentum when something it struck with it
like a high mass ratio.

[Rocky Raab]

One more thing, Harry. You can pretty much ignore both numbers. Unless there is a huge spread, any difference between two bullets using either number is pretty much meaningless.

If you're comparing a bullet with a BC of .262 versus one that is .462, you'd see a large difference in velocity loss and drop, but between a .462 and a. 492? Nada. Ditto for SD numbers.