Ok...So I am TRYING to understand "Sectional Density" and "Ballistic coefficients" . I THINK I understand the basics. Though I am ****ed if I can get through the equations I've found on BC. Or are they called "math sentences" now ? *L*

So, to sum up what I THINK I have learned....SD is the abilty of a bullet to penetrate, and in general a HIGH SD bullet penetrates more than a low SD bullet. Assuming the energy behind each bullet is equal and their weight is also equal.

Assuming I'm right..Then why would you want to use a bullet of LOW SD on a game animal ? Do lower SD bullets expand more quickly ? So 150 lb animal needs a lower SD as opposed to 800 lb animal needing a higher SD ?

BC is BASICALLY the areodynamics of a bullet...or wether or not the bullet has a high or low deceleration. So a bullet with high BC decelerates more slowly OR farther down range than one with a LOW BC. I think. Also ,a bullet with a high BC shoots "flatter" than one with a low BC, yes ? Which leads me to my next question...

Using the same caliber rifle...Lets say 308 win. For my first bullet I choose a 208gr A-max with a BC of .648. Next I choose a 180 gr rn with a BC of .241. Will the trajectory be significantly lower from the A-Max because of its BC ?

Thanks in advance

I'll take a swing at this one: You've got the basic principles of both terms correct.

SD is most important with smaller caliber bullets. When you get into 35 caliber, and higher, bullet mass helps ensure penetration. Also, the only situation where you might want a lower SD is when varmint shooting, but the expansion desired there is usually achieved with a thin-jacketed bullet at high velocity. For hunting big game, a well-constructed bullet that is medium to heavy (for caliber) will give the highest SD, which is almost always best.

BC is a bit more tricky to understand and apply to most shooting situations. Cartridges that develop fairly high velocities will actually have 2 or 3 BC's to consider, as the bullet slows down. This is only relevant for long-range target shooting of ~400 yards, or more. In the example you asked about, the 208gr bullet will start out slower but will retain its velocity/energy far better, at longer ranges. With that being said, the 180gr RN would be a better choice for big game hunting than the A-max, since trajectory will not be much of an issue with either, out to 200 yards, or so. The 180gr bullet is constructed to perform well on game while the A-max is built to be very accurate for long-range shooting. (Yes, you can hunt with it, but Hornady will tell you, this isn't what they designed it for.)

With modern, center-fire rifles of small to medium caliber, generating velocities of 2,600 to 3,100 fps, SD and BC are good to know, but if you're talking about hunting and not target shooting, neither matters as much as choosing a bullet designed for accuracy AND good terminal performance.

You're getting it but don't get too wrapped around the axle on either one. BC can and does vary quite a bit - the manufacturers sometimes get a little carried away with their claims. And it can vary when fired in one rifle or the other, and sometimes from one load to the other. Rick Jaimison did an article way back in Shooting Times that demonstrated this.

Also the G1 drag models are long obsolete BUT they are good enough to get you out to 400, 500, maybe 600 yards pretty close. You do have to shoot them at long range to see.

As far as SD goes, again, it gets hyped BUT once the bullet starts expanding everything is pretty well out the window. Don't assume a bullet with a higher SD number will penetrate deeper, as it depends a great deal on the bullet construction, cross-sectional area of the expanded shape, and sometimes just plain luck as far as what piece of the critter it hits first.

You do have the basics down - great! Just don't get caught up in the advertising hype and start believing that the bullet with .001 "better" numbers is any sort of significant advantage.....

"So, to sum up what I THINK I have learned....SD is the abilty of a bullet to penetrate, and in general a HIGH SD bullet penetrates more than a low SD bullet. Assuming the energy behind each bullet is equal and their weight is also equal."
Well, sorta. . .
Actually, SD (sectional density) is a ratio of a bullet's length to it's width (caliber)- nothing more. A bullet that's long for it's caliber will have a higher SD than a short bullet for it's caliber, which will have a low SD.
Now, back in Thee Olden Days, when all bullets were a gilding metal cup with a lead core, high SD bullets would generally penetrate more than low SD bullets.
Today, when you have bonded cup and core bullets, partition types, and solid copper expanding bullets, those rules of thumb about penetration aren't so ironclad. Penetration is a function of bullet construction as much as it is of SD.
You've got the basics off SD and BC- just don't try to take them too far.

"Actually, SD (sectional density) is a ratio of a bullet's length to it's width (caliber)- nothing more."

Well, it's a little more but Sectional Density WAS meant to predict penetration with cup and core bullets. Actually it's the realationship between diameter and weight. Thus, a military .30 cal 170 gr. FMJ-BTSP has the same SD as a 170 gr. thin jacketed flat point, flat base bullet made for a 30-30. But you can bet the farm they won't penatrate game the same at the same termnal speeds.

With the really wide variety of bullet constuction methods today the SD simply has no effective meaning.

BC does, but it's a LOT more involved and less susceptible to computation error than many seem to assume. Anyone wanting to KNOW what his bullets will do at long ranges had better check them at those long ranges.

Sectional density (SD) combines mass and area to give a figure proportional to how fast a fluid will slow a projectile relative to others the same shape. Ballistic coefficient (BC) is a multiplier used to take differences in bullet SD into account, so you can find how far a projectile travels in losing certain velocities by comparing it with a standard one.

In ballistics, SD is the ratio of mass to the square of diameter. The square of diameter, in turn, is proportional to area, always being 1.27323954473… times bigger than a round section's cross-sectional area*. This is a traditional shortcut used in ballistics, being a holdover from the day when calculations were done longhand. It avoids the extra step needed to find actual area. The oversize number still gives a result you can use to compare one projectile to another of the same shape, and rank them correctly. Everywhere else, except ballistics, SD is mass divided by actual area.

The reason mass matters to flight trajectory (or to penetration) is it is proportional to momentum. The reason area matters is it is proportional to drag, for a given shape. If you think of the bullet as a small sail being pushed into the wind, doubling its area doubles how hard it is to push through air or any other medium. Momentum determines how much force it takes to slow an object, and area determines how much force is applied to slow it. In any particular medium for any particular projectile shape, more mass makes it harder to slow, and more area slows it more rapidly.

Note the bullet shape criterion. The same area that gives a flat object a certain resistance to motion in a fluid, will give a pointy shape much less resistance. That is where, for trajectory in air, the BC comes in. It is yet another shortcut developed in the 19th century to reconcile the differences in SD and its effect on how quickly actual projectiles lose velocity in air for a given standard projectile shape. Applying that velocity loss information to the relatively simple calculation of trajectory in a vacuum, provides trajectory in air.

The problem for 19th century ballisticians was, it took firing an awful lot of rounds using their imprecise electro-mechanical ballistic chronometers to get an average result with enough resolution to get tables accurate to the nearest yard. In fact, it took many moons, so they only wanted to go through it once. They came up with the shortcut of doing this study for a standard projectile, then they could use far fewer firings of other projectiles to find a figure of merit, the BC, relative to the standard projectile's performance. This would let them predict the trajectories of these other projectiles based on the standard projectile's data. The BC adjusts the difference in velocity loss from drag between a projectile with a BC and its standard projectile.

The BC is used as a velocity loss range multiplier with the standard projectile's measured velocity loss tables. For example, suppose you've got a .223 bullet that you fire at a muzzle velocity of 3,100 fps, and you want to know how far it will travel before the velocity drops to 3,000 fps? If you look up the 3,100 fps and 3,000 fps table entries for the G1 standard projectile (the one the ammo manufacturers base their published BC's on), those tables show that it will travel 100 yards during that velocity drop. You just multiply 100 yards by your bullet's ballistic coefficient to find how far it travels dropping from 3,100 fps to 3,000 fps. If your .223 bullet has a BC of .43, then multiply .43 times 100 yards, and you get 43 yards. That's how far your .223 will travel between those two velocities.

Calculating the ballistic coefficient from the standard equations requires you to know about your bullet's drag. That is more reliably measured than calculated, though you can use the JBM on-line calculators to get close for relatively pointy shapes.


*That number is the ratio of the area of a circle to the area of a square the same diameter, which is what squaring the bullet diameter gives you.

For a different (and I think, practical) perspective regarding SD, I would invite you to check out the article at the link below:

http://www.gsgroup.co.za/articlesd.html

forget all that nerd stuff and go shoot!

Nalgi: "forget all that nerd stuff and go shoot!"

Yeah, that's what I was trying to say...!

Yeah, I haven't though this much about math since high school....MY HEAD HURTS.....LET'S JUST GO SHOOT SOMETHING!!!

Yeah, guys, but that's the purpose of this particular forum. You might be able to set your parental controls to block it from your screen? ;)