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Old April 14, 2013, 11:44 PM   #1
frakn
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bad ballistics

Starting to learn a little about ammo, and I'm looking at ballistic coefficients for various long range rifles. Is there a reason that most of the widely-used military rounds have comparatively low ballistic coefficients? Seems a little counter-intuitive for them to adopt the crappier ammo.
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Old April 14, 2013, 11:58 PM   #2
big al hunter
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I think you may be a little confused. Rifles don't have ballistic coefficients, bullets do. A ballistic coefficient is determined by the weight, diameter and length of the bullet. It determines the aerodynamics of bullets and allows us a means of calculating trajectory based on the velocity of the bullet.

The bullets used in military weapons have ammo that is good for its intended use. Most troops are doing battle at 300 yards or less, they don't need long range ballistics. Therefore 5.56 is sufficient even with low b.c. bullets. On the other hand snipers that are engaging targets at 1500 yards need the high b.c. of the 338 bullets in the 338 Lapua.

Hope this clears things up a bit
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Old April 15, 2013, 05:44 AM   #3
B.L.E.
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The shape of a bullet also has an effect on the BC. A spitzer boat tail usually has a higher BC than a flat base flat nose bullet all other things being equal.
The smaller the bore, the harder it is to get a high BC. Even the heaviest .17 caliber bullets have BCs in the 0.125 range.
The reference bullet that defines a BC of 1.00 is actually a small artillery shell with a 1 inch bore and a length of three inches if I remember correctly.

Last edited by B.L.E.; April 15, 2013 at 02:11 PM. Reason: spelling
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Old April 15, 2013, 10:59 AM   #4
g.willikers
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Then there's the idea of short range military rounds should yaw some, so they tumble upon impact.
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Old April 15, 2013, 02:52 PM   #5
B.L.E.
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Bullet stability has little to do with ballistic coeficients except that the higher BC bullets tend to be heavier and thus longer and thus tend to need faster rifling twists to stabilize.

A .177 air rifle pellet, for example, has a BC in the 0.01 range and due to the hollow tail and heavy nose, they will actually fly nose first when shot from a smoothbore air gun. Try it for yourself if you have a smoothbore BB gun and some .177 lead pellets. They always hit the target nose first at any range.

Making the bullets shorter and lighter worsens the BC but actually makes them more, not less stable.
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Old April 15, 2013, 02:57 PM   #6
James K
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A few military bullets have been designed to yaw, but for the most part it is not desirable. It is a bit silly to think that an enemy soldier shot with a full power rifle won't be bothered unless the bullet yaws or has a hollow point.

Further, an unstable bullet might produce a more severe wound in a human body, but it will not penetrate cover as well, a drawback in combat.

The military often has other concerns, not always obvious. The Army adopted a flat base 150 grain bullet with the .30-'06 cartridge. But in WWI, it was outranged by the German 8mm. So the U.S. went to a boat-tail bullet that had a longer range. Then they found out that the range was so great that bullets were striking outside the impact area of military ranges, not a good thing to have happen. So, back to the flat base bullet, which served as long as the .30 remained in service.

Jim
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Old April 15, 2013, 03:26 PM   #7
jmr40
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Quote:
Making the bullets shorter and lighter worsens the BC but actually makes them more, not less stable.
Maybe with subsonic ammo like air rifles and possibly shotgun slugs, but not with most centerfire ammo. Bullets lose their stability when their speed slows to a certain point, usually about 1,000 fps. Bullets with better BC maintain that speed much farther downrange, and are stable longer.

Short lightweight bullets start faster, but a heavier bullet with a better BC will reach a point downrange where it is moving faster even though it may have started 200 fps or more slower at the muzzle.
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Old April 16, 2013, 02:12 PM   #8
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.17 caliber bullets ballistic coefficient = >.125

http://www.hornady.com/store/17-Cal-.172-25-gr-V-MAX/

just for clarity.
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