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9/22/2017 12:11:25 AM
Posted: 4/28/2004 12:25:18 PM EDT
Okay, I've read the ammo oracle and I have one question. Hollow points and Ballistic tips, etc., are inherently more accurate due to the center of mass being farther back, right? But (according to the oracle) ball ammo yaws when it hits a watery target due to the fact that it's center of mass is aft of the middle of the bullet. I need a physicist here. Why does the same feature (center of mass being aft) stabilize a bullet in air and destabilize it in water?
Link Posted: 4/28/2004 5:24:16 PM EDT
Link Posted: 4/29/2004 3:13:17 AM EDT

Originally Posted By ballisticporcupine:
Okay, I've read the ammo oracle and I have one question. Hollow points and Ballistic tips, etc., are inherently more accurate due to the center of mass being farther back, right? But (according to the oracle) ball ammo yaws when it hits a watery target due to the fact that it's center of mass is aft of the middle of the bullet. I need a physicist here. Why does the same feature (center of mass being aft) stabilize a bullet in air and destabilize it in water?



Moving the center of mass to the rear causes greater instability in the air just as it does in the target.
Link Posted: 4/29/2004 6:22:18 AM EDT

Originally Posted By Jetlag:

Moving the center of mass to the rear causes greater instability in the air just as it does in the target.



That's what I thought. Shotgun slugs, shuttle cocks, and arrows all fly nice because the center of mass is forward. But the ammo oracle says that hollow points are more accurate because their center of mass is farther aft than a similar FMJ. This section from the oracle is referring to Ballistic Tips "The design also allows the center of gravity to be moved back, increasing in flight stability. This is the same design theory that gives hollow point match bullets better accuracy properties."

Troy - I think you're right about medium density.
Link Posted: 4/29/2004 9:44:01 AM EDT

Originally Posted By ballisticporcupine:

Originally Posted By Jetlag:

Moving the center of mass to the rear causes greater instability in the air just as it does in the target.



That's what I thought. Shotgun slugs, shuttle cocks, and arrows all fly nice because the center of mass is forward. But the ammo oracle says that hollow points are more accurate because their center of mass is farther aft than a similar FMJ. This section from the oracle is referring to Ballistic Tips "The design also allows the center of gravity to be moved back, increasing in flight stability. This is the same design theory that gives hollow point match bullets better accuracy properties."

Troy - I think you're right about medium density.



I believe it's the solid base of the "hollow point match bullet" that gives it better accuracy properties over the exposed core of the FMJ. I have no idea how moving the center of gravity to the rear could increase in-flight stability. It just doesn't make sense.
Link Posted: 4/29/2004 12:57:38 PM EDT
Link Posted: 4/29/2004 1:57:37 PM EDT
Just to post my 233rd post in the ammo forum I'll just add that an arrow is also fin stabilized and not dependent on twist rates as a rifle bullet is. Sorry if this is a lame ass post, but seems like no one mentioned it.
Link Posted: 4/30/2004 10:58:06 AM EDT
Link Posted: 4/30/2004 2:53:23 PM EDT

Originally Posted By mr_wilson:
This should help you out: www.nennstiel-ruprecht.de/bullfly/index.htm#Top_of_page

Mike



Thanks Mike. I'll have to book mark that so I can absorb it a little at a time. MMmmm, two of my favorite things; ballistics and knowledge.
Link Posted: 5/4/2004 4:08:19 PM EDT
[Last Edit: 5/4/2004 4:17:43 PM EDT by Jetlag]
Beware, I'm just a humble keyboard physicist

So, I've been doing some reading since my last post and from what I can tell, the bullet becomes more stable not because the center of mass is moving to the rear, but in spite of it. The bullet becomes more stable because you are moving a larger percentage of mass closer to the center of mass (by removing mass that is furthest out). This means that less mass of the bullet as a whole is trying to spin the wrong way, requiring less good spin to stabilize the bullet. Look up "transverse moment of inertia." But you are still moving the center of mass further and further behind the center of drag as you continue to remove mass from the nose and eventually stability will peak and then begin to drop.

So to answer the question:

Why does the same feature (center of mass being aft) stabilize a bullet in air and destabilize it in water?
It doesn't. The center of mass being aft of the center of drag would cause the bullet to tumble right out of the muzzle if not for the spin imparted to it by the rifling. It is a destabilizing factor. Like Troy explained, it's just much easier to overcome in the air.

One thing that should be noted when removing mass the from the nose... is that you are removing mass! So unless you're creating a feature like a ballistic tip or a hollow point, why do it? A shorter bullet with the same weight and profile will be even more stable and have a better B.C. Looking at a 77gr Nosler HPBT, it seems to have about 0.2" of air in the nose. If you were to try to squeeze that out of there you'd probably only shave a couple thousandths off the bullet's OAL, but hey, it'd still be more stable [and you could pack just a little more powder behind it, if that's your sort of thing].

Well, maybe it's stable enough. Perhaps Nosler got it just right for a certain twist and a certain velocity they were using. Can something be too stable? How stable should a bullet be? Just enough and no more. [Shamelessly stolen quote.] Like the ammo-oracle describes, the bullet can go nose up or even burst. (Read here for accurate descriptions of what I'm talking about.) Going nose up creates drag, and loss in velocity. As the bullet spins, imbalances in its construction will make it wobble. More spin, more wobble, less accuracy. [Wobble is a highly scientific term. ] So, why not reduce the spin and shrink the bullet? That would be better all around but who wants to buy a 1-8.2" twist barrel just to make one stinkin bullet fly better? Or maybe the bullet's construction is just a result of the machines being set up "good enough." Or perhaps a built-in margin-of-error. Whatever. Two hundredths of air or two thousandths of length or two grains of lead isn't going to mean crap to 99% of the people who shoot them anyway.

I'll stop now because it seems I've lost all notion of direction in this post. But I do believe the first two paragraphs explain a lot about what was originally asked. I've tried to be as accurate as possible but claim no lack of errors in my post. Corrections are welcome. I'll go crawl back in my hole now.
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