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Ballistic Coefficient
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Knifeboy
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PostPosted: Fri Apr 08, 2005 4:33 pm    Post subject: Ballistic Coefficient Reply with quote

I dont want to sound dumb, but I just want to know what the ballistic coefficient is? I thought this would be the bunch to ask, you guys know a lot about ballistics. Any info on this topic will be appreciated.

Thanks,
Knifeboy

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Blaine
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PostPosted: Fri Apr 08, 2005 5:49 pm    Post subject: Re: Ballistic Coefficient Reply with quote

The ballistic coefficient is a comparison number to a standard established by the military for a artillery round. This figure is set at 1.000 and all other projectiles are compared to it. Generally speaking, the Ballistic Coefficient (BC) is a figure used for comparison purposes that takes into account a bullet's shape vs sectional density & weight. The higher the number.....the better a bullet will "slip" through the air and maintain its velocity. Various factors contribute to a bullet's BC. For example the shape of the nose (the more pointed/streamlined the higher the BC) and the tail (a boat tail design produces a higher BC). All these factors added together are used to establish the bullet's BC. Don't ask about the formula used....you wouldn't want to know. Confused What it all adds up to is that the higher a bullet's BC, the more velocity it maintains down range. Hope this helps.

Blaine

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DallanC
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PostPosted: Fri Apr 08, 2005 8:31 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Very good description... I would only add that 1.0 BC is the perfect theoretical bullet... which doesnt exist in the real world. It is a value to describe how well the bullet travels through the air. You get near or above .5 BC and you have a pretty amazing bullet on your hands.


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slamfire
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PostPosted: Sun May 15, 2005 9:18 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Some folks do not agree with the calculations.

Ballistic balderdash!

Twisted Evil
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Flint54
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PostPosted: Sun May 15, 2005 11:03 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Wink BUT< the actual B.C. from your firearm may (Almost Always) not be that published by the Manufacturer. Depending on the condition of your bore, the # of lands & grooves, the depth of the rifling, the stability & condition of the bullet upon its exit, the condition of the crown and the Rolling Eyes phase of the moon, tides and who knows what else!

One very imoprtant thing to remember is the condition of the point of your bullet and the base! Those bullets that have been altered by a seater die will positively have a lower BC. If the base is damaged in any way it will also alter the BC. What I'm basicly saying is that to get the true BC you have to chronograph your ammunition at two distances, the muzzle and (I use 100 yds) and then feed the data into my PACT Chrono and it will give me a corrected BC. Most of my tests have shown a diffrence of @ .010 - .020 from published data.

Does it matter, IMO for hunting I doubt it, but for long range work most decidedly!

Just another variable in our quest for perfection! Rolling Eyes Wink
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DallanC
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PostPosted: Mon May 16, 2005 10:53 am    Post subject: Re: Ballistic Coefficient Reply with quote

The Ballistic Coefficent is a value that quantifys how a bullet travels through the air enroute to target. Of course it exists. That it is dynamic instead of being static is a different issue.


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Blaine
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PostPosted: Wed May 18, 2005 3:02 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Comparing BC's is like comparing the mileage estimates on new car stickers...it is for comparison. The number doesn't mean anything in real terms, but by comparing the BC's of bullets, you get an idea how the velocity will hold up down range. I agree that for hunting situations (in my area less than 200 yards), the bullet construction has more to do with stopping a deer than an extra 50 fps or so. But if you can have a bullet that is well constructed and still has a high BC, why not use it? Often times a more stream lined bullet (higher BC) will stabilize better and therefore be more accurate. That's not a given, but it sometimes works out that way.

Blaine

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GroovyJack
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PostPosted: Sun May 22, 2005 6:56 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Also different velocities produce different BC's ..
Lets say we have a theoretical bullet with a BC of .500 in 7mm caliber .
You launch this bullet out of yur 7 mag at 3000 fps you have one BC
You launch out of yur 7/08 several hundred fps less and you have another BC ..
As the bullet slows down the BC changes .. Also while it is true that a boattail has a better BC it does not mean the boattail is 'more accurate'
Jack

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1895ss
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PostPosted: Thu Aug 11, 2005 10:12 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Ballistic Coefficient (BC) is basically a measure of how streamlined a bullet is; that is, how well it cuts through the air. Mathematically, it is the ratio of a bullet's sectional density to its coefficient of form. Ballistic Coefficient is essentially a measure of air drag. The higher the number the less drag, and the more efficiently the bullet cuts through the air. So for purposes of flying through the air efficiently, the bigger the BC number the better.

BC is what determines trajectory and wind drift, other factors (velocity among them) being equal. BC changes with the shape of the bullet and the speed at which the bullet is traveling, while sectional density does not. Spitzer, which means pointed, is a more efficient shape than a round nose or a flat point. At the other end of the bullet, a boat tail (or tapered heel) reduces drag compared to a flat base. Both increase the BC of a bullet.

For example, a Hornady 100 grain round nose 6mm bullet has a BC of .216; a Hornady 100 grain spire point 6mm bullet has a BC of .357, and a Hornady 100 grain boat tail spire point 6mm bullet has a BC of .400. All three of these bullets have a sectional density (which is the ratio of a bullet's diameter to its weight) of .242, because they are all .243" in diameter and weigh 100 grains. But the more streamlined bullets have a higher ballistic coefficient. They are the ones to choose for long range shooting where a flatter trajectory is important.

To illustrate the practical difference between these three styles of bullets, let's use Hornady's trajectory figures for the 100 grain 6mm bullets above. Starting all three bullets at a muzzle velocity of 3100 fps from a scoped 6mm rifle zeroed at 300 yards, the trajectories are as follows.

.243" 100 grain Round Nose (BC .216): -1.5" @ muzzle, +4.8" @ 100 yards, +6" @ 200 yards, 0 at 300 yards, -15.9" @ 400 yards, -46" @ 500 yards.

.243" 100 grain Spire Point (BC .357): -1.5" @ muzzle, +3.8" @ 100 yards, +4.7" @ 200 yards, 0 @ 300 yards, -11.1" @ 400 yards, -30.5" @ 500 yards.

.243" 100 grain Spire Point BT (BC .400): -1.5" @ muzzle, +3.6" @ 100 yards, +4.4" @ 200 yards, 0 @ 300 yards, -10.4" @ 400 yards, -28.6" @ 500 yards.

There is a pretty big difference in trajectory between the round nose bullet and the two pointed bullets, making it obvious why it is folly to choose a round nose bullet for long range shooting with a high velocity rifle like a 6mm Remington or .243 Winchester. Also notice the big difference in BC between the round nose bullet (.216) and the spire point bullet (.357).

But there is less difference between the trajectory of the flat base spire point bullet and the boat tail spire point bullet. The boat tail helps, but not nearly as much as the point on the front of the bullet. The boat tail bullet had .3" inch less rise at 200 yards, and 1.9" less fall out at 500 yards. These differences are real, but unlikely to make or break a shot at a big game animal. This is shown by the smaller difference in BC between the two pointed bullets, .357 for the flat base and .400 for the boat tail.

To further assess the importance of a boat tail, note these pairs of Speer spitzer bullets of the same weight and caliber. In each pair, the first bullet has a flat base, and the second has a boat tail.

.243" (6mm) 100 grain, BC .351
.243" (6mm) 100 Grain BT, BC .430

.257" (.25) 100 grain, BC .369
.257" (.25) 100 grain BT, BC .393

.277" (.270) 130 grain, BC .408
.277" (.270) 130 grain BT, BC .449

.308" (.30) 165 grain, BC .433
.308" (.30) 165 grain BT, BC .477

A list of pointed (spitzer type) hunting bullets considered a good bet for long range shooting in their respective calibers, with their ballistic coefficients, follows. Boat tail bullets are designated "BT," all other bullets have flat bases.

All of the figures that follow are taken from the Speer Reloading Manual Number 13. The same weight bullets from other manufacturers will have different BC's, because they are slightly different shapes. But these Speer numbers are as typical as any, and being from the same source they are useful for purposes of comparison.

.224" (.22) 55 grain, BC .255
.243" (6mm) 90 grain, BC .385
.243" (6mm) 100 grain BT, BC .430
.257" (.25) 100 grain BT, BC .393
.257" (.25) 120 grain BT, BC .435
.264" (6.5mm) 120 grain, BC .433
.264" (6.5mm) 140 grain, BC .496
.277" (.270) 130 grain BT, BC .449
.284" (7mm) 145 grain, BC .457
.308" (.30) 150 grain BT, BC .423
.308" (.30) 165 grain BT, BC .477
.311" (.303) 150 grain, BC .411
.323" (8mm) 150 grain, BC .369
.338" (.338) 200 grain, BC .448
.375" (.375) 270 grain BT, BC .429

Note that with bullets of the same weight and style, such as the 100 grain .243" and .257" bullets or the 150 grain .311" and .323" bullets, the smaller diameter bullet always has the superior BC due to its better sectional density.

As examples of very streamlined bullets, note the BC of these Speer match type bullets. These are all pointed hollow point, boat tail bullets.

.224" (.22) 52 grain Match, BC .253
.284" (7mm) 145 grain Match, BC .465
.308 (.30) 168 grain Match, BC .480
.308 (.30) 190 grain Match, BC .540

Note that in the .308 pair, the heavier bullet (which has the greatest sectional density) has the better BC. The extremely poor SD of the .224" bullet lowers its BC, even though its shape is similar to the others.

Which explains why .22 bullets drop so much at long range and are so subject to wind drift, compared to larger caliber bullets with superior sectional densities (and hence, BC's). At a MV of 3100 fps a .224" Speer 52 grain BTHP Match bullet zeroed for 300 yards has a 500 yard drop of -43.9 inches, not much better than the 6mm round nose bullet in our trajectory examples near the beginning of this article. An interesting subject, this ballistic coefficient, and worth paying attention to when you select a bullet (or a caliber) for long range shooting.

I hope this helps Smile
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1895ss
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PostPosted: Fri Aug 12, 2005 7:45 pm    Post subject: Re: Ballistic Coefficient Reply with quote

By the way, I didn't write the article I posted. I got that from somewhere years ago and I thought it was a pretty good example, that's why I kept it and posted it here.

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delboy
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PostPosted: Tue Aug 16, 2005 6:15 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Hey Flinty, what's the BC of the stones and nuts & bolts that you shoot?
Seriously though, does anyone have any clues as to where I can find the BC figures on airgun pellets? I contacted H&N in Germany but they didn't grace me with a reply. I have a ballistic table for air rifles but without the BC figures it ain't worth Jack......... I can't seem to find anything covering pellets on the net, so any leads would be gratefully received. Thanks guys,
Del

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Blaine
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PostPosted: Wed Aug 17, 2005 6:29 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Based on the shape and weight of air rifle pellets, I would say the BC would be so low as to be un-useful. The shape of most pellets I've seen are not very "sleek" compared to rifle bullets.

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delboy
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PostPosted: Thu Aug 18, 2005 4:48 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Maybe not Blaine, but they take some beating for sheer accuracy - will any of your rifles CONSISTENTLY make one cloverleaf shaped hole in a target at 50yds. ? Can you light a match at 20yds. without breaking the wood? Hmmm........ (Not that there is much point in doing either - just fun shooting) I can also take a crow or a woodpigeon with a headshot at 60yds. which I don't think is bad for a "puffer".

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