AR Sponsor
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? |
|
Because the water is hundreds of times more dense than air. The denser the medium that the bullet is traveling through, the faster it would have to spin to remain stable. There is no way to increase the spin rate enough to make it stable in water or flesh. The rifling would look more like bolt threads, with 50+ revolutions per inch, instead of 1 revolution per 7 or 9 inches. Obviously, that wouldn't work. But, also note that one of the other reasons these bullets are more accurate is because the bullet base is jacketed and therefore is very uniform. Any lack of uniformity at the base will cause a loss of accuracy as the bullet leaves the barrel and the gasses blow past the bullet base. -Troy |
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. |
|
It is true that moving the weight to the rear of the projectile helps it in long-range flight, but that's only true due to the high speeds that the bullet travels. If you look at a modern jet, you'll notice that they've moved the weight to the rear of the plane, which makes the plane unstable. That's why they require computer control of the control surfaces. The result of all this is that the plane can maneuver and handle better. A dart or an arrow doesn't have nearly as much velocity, and is much more susceptable to winds and such. Flight modeling is pretty advanced physics, and I'm not an expert, but I've done some reading and research for myself. -Troy |
|
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. |
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:
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. |
AR Sponsor