BC: and SD: in reloading tables?

[rrockefe]

I recently started preparing to reload after a hiatus of many years. I bought a Lyman Third Edition Pistol & Revolver handbook. I'm not familiar with the subject nomenclature that is located in the top right corner of all the tables. I don't remember this being displayed in my old manuals. Would someone please explain to me what these mean? I've been all through the manual and there is no explanation listed.

 

[rrockefe]

I recently started preparing to reload after a hiatus of many years. I bought a Lyman Third Edition Pistol & Revolver handbook. I'm not familiar with the subject nomenclature that is located in the top right corner of all the tables. I don't remember this being displayed in my old manuals. Would someone please explain to me what these mean? I've been all through the manual and there is no explanation listed.

 

[Noah Zark]

Blatantly copied from Wikipedia:

In ballistics the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant "i" that relates to the aerodynamics of the bullet's shape. Ballistic coefficient has units of lb/in² or kg/m². Normally BC's are stated in lb/in² by gun projectiles producers without referring to this unit.

Sectional density is the ratio of an object's weight to its cross-sectional area. It conveys the ability for an object to overcome resistance. When a projectile is in flight or impacting an object, it is the sectional density of that projectile which will determine how efficiently it can overcome the resistance to air or object. The greater the sectional density is for a projectile the greater its efficiency is and therefore ability to overcome the resistance of air and object. Sectional density is stated as Mass divided by cross-sectional Area calculated on the bullet's diameter, with units of kg/m2 or lb/in2.

Noah

 

[rrockefe]

That was quick! Thank you very much!

 

[WR Moore]

Short version: BC is used to calculate ballistic tables, the higher the BC, the better the bullet retains velocity. SD is used to evaluate comparative penetration between different bullets. The higher the SD, the greater the theoretical penetration-expansion is not considered.

 

[Alk8944]

Originally posted by WR Moore:
Short version: BC is used to calculate ballistic tables, the higher the BC, the better the bullet retains velocity. SD is used to evaluate comparative penetration between different bullets. The higher the SD, the greater the theoretical penetration-expansion is not considered.

Not really. All Sectional Density indicates is relative weight per square inch expressed in pounds. All bullets of the same profile and length have the same Sectional Density, regardless of caliber, so long as construction is identical. Likewise, all bullets of the same diameter and weight have the same SD, regardless of shape. Penetration is effected to a degree by this figure, but it is a result not any sort of indicator.

If you want to check this find standard for caliber bullets of several calibers and having the same shape, such as 158 SWC for .38, 210 SWC for .41M, and 240 SWC for .44. What you will find is they are all very close to the same length, and checking the manufacturers specs you will find they are virtually identical in SD too. Ever wonder why different calibers have certain weight bullets as "standard"? Basically this is the answer.

 

[Rule3]

Generally speaking with all other things equal, is a bullet with a higher BC more accurate than a lower one??

 

[WR Moore]

BC has nothing to do with accuracy-other than that a bullet with a high BC may be too long to properly stabilize in your rifling twist.

Bullets with high BC's are generally match grade bullets, which does make a difference, but the accuracy is a result of their being match grade bullets.

Alk8944- your last paragraph expresses what I tried to say. SD allows comparison of bullets of different diameter, weight and design. Higher SD does play a part in penetration or we wouldn't use depleted uranium in anti-tank rounds (MUCH higher SD).

 

[canoe on the yukon]

SD is sectional density......it's weight in relation to diameter.

BC is ballistic coefficiency....it's shape and otherwise aerodynamic qualities.In other words,it's relative ability to retain velocity and hence have a flatter trajectory (when all else is equal).It has nothing to do with accuracy.

 

[Gun 4 Fun]

Given identical design/construction and caliber, a bullet with a higher sectional density will always out penetrate one with a lower sectional density. Sectional density is a very good way of comparing penetrative qualities of bullets with like construction.

Noah's post above bears that out. It has been used for years as the standard for comparing projectiles.

The BC's published in bullet company brochures and manuals are not absolutes. They advent of Oehler's Model 43 started helping writers and the general population that could afford them see that. Very often a BC published will be off quite a bit from what the bullet turns in when actually fired across the screens and the calculations are done. They are usually figured by using two seperate units, one near the muzzle, the other at 100 yards. The velocities are recorded and calculations made that give the actual BC. BC also changes with velocity, atmospheric conditions, etc.