Bullet Ballistics

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Glass Fractal
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Bullet Ballistics

Postby Glass Fractal » Sat Apr 02, 2011 5:24 pm UTC

Why is it that a bullet is more stable in the air when its weight is shifted toward the base? ( It seems like the opposite would be true, that the heaviest part should be in the front, but when match grade bullets (ones where the only consideration is accuracy) are made they have a hollow point. That doesn't seem like it would improve aerodynamics either since the shape is still the same.

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Re: Bullet Ballistics

Postby Yakk » Sat Apr 02, 2011 7:08 pm UTC

Hollow point also stops better (ie, doesn't penetrate targets as far). That could be the primary reason why they have hollow points?
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EdgarJPublius
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Re: Bullet Ballistics

Postby EdgarJPublius » Sat Apr 02, 2011 7:17 pm UTC

Having the center of gravity behind the center of pressure actually does have a de-stabilizing effect which has to be countered by spinning the projectile about it's axis.

This is why at long ranges, small arms projectiles tend to flip end for end or 'keyhole' as rate of spin slows during flight until it no longer provides sufficient stabilization to overcome this tendency.

This effect is sometimes exploited by military rounds, sue to the Hague Convention, expanding ammunition is prohibited in military conflicts, so often rounds are designed to be very rear heavy to improve the tumbling effects of the round in flesh and get a similar result.


According to Wiki, moving the CoG rearwards can also improve the ballistic coefficient. I'm not entirely sure why that is though, but it would account for bullets that are strictly less stable still being more accurate.
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Re: Bullet Ballistics

Postby p1t1o » Mon Apr 04, 2011 1:04 pm UTC

Bullets spin very fast indeed, so shifting the weight to the rear of a round will not affect stability too much.

Small arms rounds, especially rifle rounds which are longer and thinner, do tend to tumble, but mostly on penetration of the target (the rifling is effective, they fly straight) as the lighter nose slows down faster than the heavier base. There have been issues in the past with lighter rounds NOT tumbling on penetration at range, if their speed drops too much, limiting their wounding capacity. A round may tumble in flight, but by this time it has flown very far and is going pretty slow for a bullet.

There is however, such a thing as a "ballistic cap" which is used to make an unaerodynamic payload (eg: a shaped charge) have a better aerodynamic profile.

@Yakk
Yeah, I would say this is the most likely reason. Even without a hollow point, the centre of mass of a long-rifle round is significantly beind the centre of pressure.

There is no term in the calculation for ballistic coefficient to account for where the centre of mass/gravity appears. However, muzzle velocity will be increased slightly for a hollow-point due to its lower mass, and this could contribute to accuracy.

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Re: Bullet Ballistics

Postby gorcee » Mon Apr 04, 2011 8:08 pm UTC

The charge is located behind the slug. So, you need a denser material in the aft section of the bullet to maintain the structure on firing.

Also, the nose of the bullet is, of course, smaller in volume. So weighting it would require a material of substantially higher density.

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Re: Bullet Ballistics

Postby Izawwlgood » Mon Apr 04, 2011 8:12 pm UTC

Hollow point rounds are designed to cause maximal damage to fleshy targets as well. Their flight stability or lack thereof is not their primary design feature.
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Re: Bullet Ballistics

Postby p1t1o » Mon Apr 04, 2011 9:07 pm UTC

Izawwlgood wrote:Hollow point rounds are designed to cause maximal damage to fleshy targets as well. Their flight stability or lack thereof is not their primary design feature.


The OP mentions "match grade" ammunition which is primarily intended for competitive shooting. Although military marksmen often use "match grade" ammunition, this is usually the first few batches of regular ammunition whenever a mold is replaced (finer tolerances in bullet shape and composition).

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Re: Bullet Ballistics

Postby Cobramaster » Tue Apr 05, 2011 1:23 am UTC

Also some bullets especially the BMG .50 employs a boat tail design to move the CoG slightly forward and to reduce the effects of wind on bullet drift. The nifty thing that pops up is drift caused by the spin of the bullet which at long range forces the bullet in the direction of the spin.
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Re: Bullet Ballistics

Postby EdgarJPublius » Tue Apr 05, 2011 4:44 am UTC

The main advantage of a boat tail is it decreases drag by reducing turbulence at the base of the bullet.

The really nifty thing that pops up is that bullets are subject to the magnus effect where spin, combined with a crosswind can cause the bullet's trajectory to deviate up or down depending on the direction of the wind vs. the direction of spin.
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gorcee
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Re: Bullet Ballistics

Postby gorcee » Tue Apr 05, 2011 3:41 pm UTC

p1t1o wrote:
Izawwlgood wrote:Hollow point rounds are designed to cause maximal damage to fleshy targets as well. Their flight stability or lack thereof is not their primary design feature.


The OP mentions "match grade" ammunition which is primarily intended for competitive shooting. Although military marksmen often use "match grade" ammunition, this is usually the first few batches of regular ammunition whenever a mold is replaced (finer tolerances in bullet shape and composition).


Right.

Again, to address the OP's question (specifically, why are match-grade bullets hollow point), I would argue that it's most likely because: a.) hollow point = lower mass = higher velocity = less time on target = less drift due to spin, b.) hollow point, because you can have a hollow aft, because the rear part of the bullet has to withstand the acceleration due to the powder charge and possibly c.) it leaves a better, more visible mark on the target so the shooter can adjust her next shot.


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