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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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I believe the phenomenon referred to above is called spherical predecession (if I got the spelling right), and was the subject in a Precision Shooting magazine or two. Yes, too much spin does cause issues with a highly arching bullet because the nose does not point back down at the angle it should when the bullet heads back to earth. In the example that the magazine cited, it was 155gr. Palma match bullets from a .308 at 1,000 yards if memory serves. 1-13" twist rate is I think correct for that bullet. With a faster twist there are problems at the 1,000 yard line.

The slight difference in twist rates could cause a bullet to stay out of the transonic zone at 1,000 yards. While the example cited was for a full 1,000 yards, the distance was less important than the fact that the bullet was starting to drop to near the speed of sound.

So it could occur at lesser ranges, or greater. But with modern bottleneck rifle cartridges and spitzer bullets, it's ordinarily going to be beyond 300 yards and most likely well over 600 yards. Definitely to be ignored at typical hunting ranges.
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Okay, so imperfections in the bullet are amplified more the faster it spins. Makes sense.

What I am now having questions about is bullets pointing nose down at longer ranges. The Marine Corps has always taught me that a bullet stays at the same angel as it’s fired. And it sounds like a few of you are suggesting that a bullets nose drops at longer ranges and enters the target the same as it would 100m. So if you fellas could clarify if you are saying that of not. And if so, how does that work.

by the way, thanks for the to the articles MZ5.
If that was universally true for all projectiles, you couldn't hit anything over the horizon with artillery. Think about high-angle projectiles (mortar rounds, howitzers, etc.). If the rate of spin in the projectile is correct, then the nose comes over at the correct angle on it's way down. If not.... then yes, you run into problems.

Rifle bullets are no inherently different than cannon shells in their flight characteristics.
As a practical matter yes, but the instructor was wrong, to be blunt. If bullet noses stayed pointed at the same angle up it would be nigh impossible to hit targets at very long range. Forget about spitzers - have you seen long range black powder shooting? Those trajectories are quite arched indeed. They would soon be tumbling and going every which way if the nose stayed up, say at 5 degrees, as the bullet was coming down at 10 or 15 degrees. Yet those shooters can hit stuff 1,000, 2,000 yards, and beyond. Not easy but possible.

The bullet nose does not necessarily point back down at the EXACT angle that the bullet is headed to earth at. If I read the article correctly, it lags the angle somewhat. The amount of lag is the issue. Too much difference (caused by too much spin on the bullet), and then the drag goes up. It is true that the effect it minor inside of 1,000 yards, but it does exist and proves the point. Otherwise it would matter not in the least what the twist rate was when shooting a the 1,000 yard mark. Sorry but I forget the exact mechanics of how the nose gets pointed down to follow the trajectory.

It was interesting that the author was not drawing on experience from ballistics or the shooting sports, but knowledge of orbital flight paths to figure this out. Subtle stuff indeed and not Riflery 101 for sure.

For specific cases, at specific ranges, the effect of all of this may not matter one bit. A bullet spun too fast (standard weight in a non-standard twist, ie, 55gr. in a 1-7" .223 barrel) will keep the nose up excessively. Even then, the nose will start to be pointed down along the bullet path. Just not an ideal amount. But to say that the bullet will keep the exact nose angle all the way to the target is not correct except at extremely short ranges. Once it goes to sleep then the nose does not vary right or left much, but will in fact start to pitch over, slightly, as the trajectory arches down.

For the average Marine Corp drill instructor, giving information to the average recruit, whether the answer is correct or not is of no practical consequence. He could have told his recruits that the bullets spin end over and and you could still go out and shoot the qualification course. You have to be in a long-range competitive shooting sport for it to matter. Even then the shooters may not know or care. They just use what works.

Sorry if this comes across as nit-picking.
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Anybody shoot 1,000 yards? If so, are the bullet holes round or oblong, and if oblong, in what direction?
I figured maybe a .45-70 with a 500 grain bullet would start to make an oblong hole, if the nose didn't follow the trajectory back down.

Anyway.... think I finally have a clear idea in my head why the bullet noses follow the path back down. Bear with me.

Let's say you are using a long pointy cone for a projectile. Instead of launching it point-on, say it's sent downrange with the nose up some ridiculous amount, 45 degrees. And some spin to keep it stable (spin being around the long axis).

Now.... given that there is a lot of cross-sectional area at the base, dropping down to none at the nose.... it might be reasonable to think that the base will have more drag than the nose. And ever so slowly, the base will get pushed behind the nose.

How fast will this happen? Depends on the rate of spin. Just barely enough spin to keep it stable.... the base will get pushed back behind the nose pretty quick as there's little angular momentum to keep it in place. Ramp up the spin to 100,000 rpms (ignore that there could be defects in the cone that might make it not spin true). Result? Lots longer till the drag on the base can overcome the angular momentum and push it back in line. Assume identical forward velocity for both examples.

So.... the higher spin rate on the second cone ends up making it go through the air sideways longer, and ultimately more drag. It slows down faster going downrange and arrives at the target slower.

Ever notice that a beautiful tight spiral on a football heads back to earth point first?
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I do agree that it is a small thing, you are absolutely correct in stating that. It only does come into play at extremes of range and trajectory, per the example with 1,000 yard shooting and the 155gr. Palma bullet and the two different twist rates.

While some might view it as trivia, personally I found the subject quite interesting. To each his own!
It does seem odd that a fairly light bullet, such as the 155gr. Palma, can shoot to 1,000 yards yet a heavier bullet such as the 168gr. MatchKing cannot stay supersonic to that range. Guess there are some very subtle things about bullet design that are not obvious to the common folk..... ?
I thought some shooters were reporting tumbling beyond 600 yards with the 168gr. bullet?
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