Within External Balistics Please explain BC (Balistic Coefficient)

[bbogue1]

I shoot pistols. I am thinking of getting into the world of rifles, so, I'm doing some studying. I understand the relationship between bullet weight, trajectory and powder use, to keep the pressures under the chamber design limits. What I don't understand is selecting a bullet to meet my goal. Could you give me a hand figuring out bullet and powder selection? If this is in the wrong place feel free to move it to a more appropriate section.

[ryanmattes]

Ballistic coefficient is about bullet shape to mass ratio. It's about the front-on cross section, versus the momentum it carries.

If you want to do the math, it's the mass of the bullet divided by the forward-facing cross-sectional area times the drag coefficient.

If you just want to pick a bullet, a higher BC is better.

You probably don't need to know much else, unless you're designing a bullet. If you're comparing two bullet profiles, check the packaging. The one with the higher BC is better at resisting drag.

Here's an estimator for cast bullets, if you want to play around. I'd recommend reading the instructions first.

http://tmtpages.com/calcbc/calcbc.htm


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[Rcmaveric]

BC is the basicly the aerodynamic efficiency factor.

The higher the BC and the slower the velocity drop of X amount of distance. Also cross wind deflection is lessened.

All things being equal in a perfect world, if two bullets fired at the same time with same 5 mph cross winds with the only difference is BC. The bullet with higher BC will have less bullet drop and less crosswind deflection.

Now comes the tricky part. It takes a good distance for those factors to become apparent. You can use a ballistic app to get a graph visualization. Around 200+/- 50 yards (depends on velocity but with my castbullet speads of about 1850fps this is the zone) is where you will see the affects of BC enough to be able to tell. But its still small. Once you start getting out past say aroun 300 to 400 yards it can make a big difference.

There are things you can do to compensate. Like use a heavier bullet to maintain velocity (mass in motion, more mass means it holds the motion). Or higher speeds (thats why those 223 guys swear by 3000fps and how flat it is). Higher spead will you give you a few extra feet before it drops like a rock but it still lacks mass to maintain that motion.

There is a lot more to external ballistics than just meer BC. Think of it as more like a tie breaker and not a deal maker. Inside 150 yards it doesn't matter much. Flat point, spire point or LFWN they will be similar.

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[cwtebay]

I think of BC as a retention factor. Since we can't beat gravity (a dropped bullet from your hand will drop at the same rate as a fired bullet - no matter the forward velocity) we want to maintain speed as long as possible when the projectile is in flight. The higher the BC, the more speed that projectile will retain at a longer distance.
i.e. a 30 caliber bullet with a higher bc will be going faster at the same distance as a lower bc bullet, which will result in less drop from the point at which it was fired from (muzzle height).

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[country gent]

More info is needed for selecting a bullet, Hunting, big game, varmints, dangerous game. Target, formal or informal. velocity range and caliber of cartridge.
I shot 243 in long range matches but it was a custom built rifle with a 1=7 twist barrel. The 115 grn berger vlds started out at 2700 FPS reach the target at 1000 yds still traveling around 1600 FPS or a little better. A 308 a 168 match king from my service rifle started out at 2600 FPS and would be subsonic at 1000 yds. Rhis the difference in the ballistiic coefficient between the 2 bullets.

Hunting requires considerations of expansion also among performance. Again caliber velocity have a big part in this. In 308 or 30-06 a 150 grn may give great results at the 2600 FPS of these cartridges. But load the same bullet in one of the magnums at 3100 and expansion may become explosive and penetration will suffer. Varmints being thin skinned need a lightly constructed bullet here you want explosive expansion. This also cuts down on the chance of ricochets.

Targets shooting range and or velocity are the big factors. Here accuracy and range are important. 100-300 yds most will work fine getting out farther 400-1000 bc becomes important both for velocity retention and wind resistance. The vlds are very good but are longer for a given weight so you need the right twist barrel to stabilize them.

Looking at the manufacturers description of a given bullet will give good information as to use, velocity range, BC, and what to expect.

The thing is to "learn" your rifle and calibers performance. Knowing the drop and wind at different ranges allows you to "compensate" for these. The old Black powder cartridges like 45-70 loaded with a heavy lead slug at 1100 -1200 fps will perform admirably at 1000 yds in the right hands.

[popper]

It's about trajectory, period. Higher means flatter shot. Short flat nose has lower BC, long skinny and pointy nose is higher. Usually somewhere around 0.3 for rifle hunting bullets. Several ballistic calculators on the interweb, grab one, put 0.3 in (with bore, bullet length, weight) and see the drop at range. Then shoot some and see how you compare. Adjust the BC in the calc. till they match. Then you will know the 'rise' or drop for the range you shoot. It will be approx as the calc doesn't take into consideration the 'recoil' effect on YOU.

[Duckiller]

What are you going to shoot and how far away will it be? A moose in heavy brush needs a big fat flat bullet. an Antelope at 300 yards needs a long skinny bullet that will expand at what ever speed it has when it hits. Pick a caliber then decide what bullet you want. Cup and core, solid copper, partition, and other assorted projectiles all work for what they are intended for. You get to decide. I would suggest cheaper bullets but of good quality for extensive target practice and expensive bullets for hunting. Nosler partitions really work as doSwifts and Barnnes. Hornady and Speer best bullets also work. Shoot enough of them so you know how the will behave in your gun before you go hunting. A 22 with an action similar to your center fire gun would be good for cheap practice. Enjoy your rifle experienve.

[Daekar]

I think the others have more or less given you what you need to know, but I'll put in my take on things just for the heck of it.

First, when selecting a bullet to meet your goal, you have to precisely define, if possible, what that goal is. For instance, "I want to go deer hunting" is a place to start, but "I want to hunt the small deer in the Southern states in a wooded area where shots beyond 150 yards are unlikely," or "I want to hunt prairie dogs in the plains states," or "I want to take up long-range target shooting" are all better.

As far as defining BC, you can think of it as how well a bullet slips through the air. If a bullet has a poor BC, then it will experience more drag and lose more speed as it travels through the air. If it has a good BC, then it will experience less drag and it will lose speed slower.

All things being equal (they aren't really, but...) for a bullet of a given diameter and shape, the heavier it is the more likely it is to have a nice high BC. The problem, though, is that a heavier bullet also takes more energy to push to a given speed. BC is great and all, but if you can't push the bullet fast enough to begin with then you might be better off with a faster bullet.

Cartridge designers know this, of course, as do competitive shooters. That's why you'll see so many cartridges nowadays with skinny, heavy-for-caliber bullets that are propelled by a case just large enough to achieve the velocity the designer is going for.

To a certain degree, brute force can be used to overcome the disadvantages of a short, lighter bullet with a poorer BC. For instance, some of the faster 22 caliber cartridges can push bullets over 4000fps! That is crazy fast, and it means that at least for a while, the bullet is moving so stinking quick that even though it's bleeding off energy rapidly it still covers a good distance before gravity can pull it down very far. If you are dealing with very long ranges though (and the definition of long varies) you are usually better off settling for less muzzle velocity with a bullet that doesn't slow down as quickly.

I do all my shooting inside 100 yards, and honestly the BC of my boolits doesn't matter a hill of beans, pistol or rifle. I've got a mold which throws bullets that almost look like a squat little coke can, and they are just fine. If I were picking a rifle for longer shots, I would pick one designed around the heaviest bullet for the cartridge which doesn't produce ridiculous recoil. This usually means something in the .243 Winchester area, although there are probably more modern cartridges that are easier to get with barrels of the correct twist for heavy bullets.