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Question regarding "over-stabilization" and twist ratios...

14625 Views 38 Replies 8 Participants Last post by  unclenick
Kind of a technical question, but...

I understand that a barrel has to have a sufficient twist ratio in order to stabilize bullets in flight. Other than a bullet coming apart and fragmenting mid-flight, how is a bullet "over-stabilized".

Example... Let’s say one 308W bullet stabilized with a 1-10 Twist, and another is stabilized with a 1-12 twist. Why would it hurt stabilization and accuracy if you lowered the twist of both rounds as long as they don’t fragment mid-flight?

My understanding is that different gr bullets need different twist ratios due to their length. And it makes sense. But it doesn’t make sense that a bullet spinning too fast can be less accurate. (As long as it doesn’t come apart)
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Voodoochild, you might be interested in some of the articles written by Bryan Litz, who is now the ballistician for Berger Bullets. You can find some of them at:
The ones there are freely readable, and I believe that at least one directly addresses your question. The way I recall it, he says something fairly close to 'don't worry about it,' but perhaps I'm not remembering properly.
Voodoochild, the Marine Corps instructed you correctly, at least as far as practical application to modern/current infantry riflery.

First of all, remember that spin stability increases as a bullet travels downrange. Next, have a look at the "Spin and Coriolis Drift" article on the page I linked. Note that if a bullet 'nosed over' vertically in precise alignment with its velocity vector (precise direction of travel at any given instant) all the way through its flight path, it would also turn its nose to the right or left, depending upon whether your rifling is right-hand or left-hand twist. Because gyroscopic stability increases as the bullet travels further downrange, this nosing left or right would get worse and worse as the bullet noses over further and further downrange. So, you'd lose aerodynamic efficiency for that reason even as you were 'gaining' it by having the bullet 'nose over.'

Litz has written/shown (I think in one of his articles on the linked page, but perhaps I saw it elsewhere?) that the impact on trajectory resulting from the amount of increase in aerodynamic drag on a rifle bullet from the bullet failing to nose over, is quite tiny indeed.

I'd think about bullet mass symmetry as mentioned above, but I wouldn't bother with the issue of nosing over, especially since there are countervailing consequences as the bullet does so.
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Not at all, MikeG. I think it's interesting to discuss how the specific technicalities bear upon the reality. My point was that the specific technicalities don't appear to bear on reality too much for small arms, and to the extent they do, there are additional and sometimes countervailing specific technicalities to consider.

I'd not be too quick to say that the author's (Bryan Litz) comments are not drawn from shooting experience. He's a national champion competitive shooter and the current head ballistician for Berger Bullets. He formerly worked in this field for the military as well. So I think he has significant practical experience to augment his formal and theoretical education/experience.

Again, a 'technical' point to remember is that nosing a spin-stabilized bullet over causes it to nose sideways at the same time. So, it's something of a trade-off because vertical nosing over gains you a minimal amount of aerodynamic improvement, which is counteracted by a minimal amount of aerodynamic decline from the nosing sideways. I believe that unclenick has also posted here before how bullets actually run downrange at a very small up-and-sideways angle due to these forces? Please correct me if I'm mistaken about that.

The long-range shooting you mention might be a bit misleading? The bullets don't tumble because they can't. If the bullets are stable when they leave the barrel, and so long as there's no trans-sonic disruption severe enough to cause tumbling, then they become more and more gyroscopically stable the further downrange they get. So they'd never be able to tumble; they're too well stabilized. Failure to nose over, or to nose over completely, simply reduces BC a touch, and thus the trajectory is slightly worse than our ballistics programs tell us. Litz addresses this point in one of his freely-readable papers, though perhaps it's not at the exact link I provided above.

You can account for all these various forces, attack angle vs. trajectory discrepancies, and flight path effects, and the military does exactly that (mostly with larger guns and projectiles). It's just that there may not be enough reason to calculate all that out for small arms? We make corrections for these and other factors via sighters, etc., but we don't crunch the actual numbers except as a hobby or educational exercise (which can still be lots of fun! :) )
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I agree that such a bullet as you describe will 'turn' via air pressure to put the pointy end forward. I realize that I used an 'if' rather than an 'as' in an earlier post, and that was a poor and incorrect choice of word. I've now corrected it.

I don't argue that a bullet fails to have its nose re-directed by air pressure. I argue that the aerodynamic drag change due to how much a bullet noses over, and how much different that nose-over and consequent drag change is in, for example, a 10-twist 308 vs. a 12-twist 308 is veryy small, such that it can effectively be ignored, IMO. And as to the different magnitude of the overall gyroscopic drift resultant from the 2 different twist rates, that's not something to be calculated, it's something to be measured in the field and dialed out with your sighting mechanism.
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Well, again, I suggest taking a look at a Springfield '03 ladder sight. It can't quite be ignored. A foot at 1000 yards is roughly equal to 1 mph wind drift.
I believe you're talking about the total gyroscopic drift from a 10-twist '03, whereas I'm talking about the difference between a 10-twist and a 12-twist, which won't 'make' you miss a man-size target at that range (especially since the sight compensates for you as I recommended :) ).
I am curious, though, to see if we don't start to hear of some 12.5" twist Palma tubes showing up with the new, longer shape .5 BC BC Palma bullets from Sierra and Lapua? Time will tell.
I am curious about this as well. I almost picked up a box of the 155.5 Berger Fullbore bullets yesterday, but since I'm not a professional Palma shooter, over $40/box was too rich for my blood. :) I do believe I'll order a box of the Sierra 2156s, though, just for fun. With the aerodynamic advances in 155-grain bullets, can similar advances for the 175-class bullets be far behind, or is there not a particular driver for such advancement?
While some might view it as trivia, personally I found the subject quite interesting. To each his own!
Hope you don't think I fall into the trivia camp, MikeG. I find it terribly interesting, even though I don't think about some things at all when my brother and I are out shooting things far away. :D
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