Rifle Twist Question

[Plain Ole Dan]

What happens to a bullet, say a 52gr .223Rem, out of a faster twist barrel, like a 1in7? I have heard of the term 'over stabilized' but never understood how a projectile could be over stabilized. Could someone please enlighten me?
Thanks,
Dan

[CENTERMAST]

What happens to a bullet, say a 52gr .223Rem, out of a faster twist barrel, like a 1in7? I have heard of the term 'over stabilized' but never understood how a projectile could be over stabilized. Could someone please enlighten me?
Thanks,
Dan

I can't explain it any better than the folks at Nosler so her goes from their #6 reloading guide: "The faster a bullet goes, the easier it is to stabilize. It doesn't spin any more than what the twist rate is in the barrel, it simply generates more rotations per second. For example, a bullet coming out of a 1-10" twist barrel will make one complete revolution for every ten inches it travels, even after it leaves the barrel, regardless of velocity. A bullet that is traveling at 3400 FPS will cover ten inches much more quickly than one traveling at 2400 FPS, and it can be said that the faster bullet is 'spinning faster', but it is simply cranking out more RPM and still not turning more than once every ten inches."

O.K. now part II

"A very common misconception is "overstability." Bullets do not become 'destabilized' or 'overstabilized' with faster twist rates, or greater velocities. They may not be quite as accurate as they were with slower twist rates or velocities, but they are stable. An unstable bullet may actually tip or tumble on its way to the target." Hope this helps.

[Don T.]

I'd never heard of it until now, but found a tutorial on the Fulton armory site. http://www.fulton-armory.com/fly/stab.htm

[Tomcat088]

Yes, bullets can be "over stabilized". I'm pretty much going to repeat what I've said in other threads. In case you care to read those as well, here's some links. http://forum.pafoa.org/rifles-42/422...-tactical.html
http://forum.pafoa.org/general-2/290...e-explain.html
http://forum.pafoa.org/general-2/290...ast-twist.html

"Over stabilization" comes in many different flavors, and can have varying effects. Lots of times it's related to how fast the bullet is moving and how fast the twist is. In the most extreme (and rare) cases, bullets are sometimes spinning so fast that they will actually explode before they hit the target. You will just see a puff of smoke or pieces of bullet jacket scatter on the ground before the target. You will NOT see this in .223 though, as the velocities aren't fast enough, but it does happen sometimes when pushing light bullets WAY too fast in hotrods like the .22-250 in a fast twist.

The other thing that sometimes happens is that the bullets don't "go to sleep" until further down range. It should be noted that there's lots of people that don't think the phenomenon I'm fixing to mention is related to twist rate, and merely a fluke, others believe it's related to certain bullet designs and twist rates. There are times when no matter how hard someone tries, they can't get a bullet to group tight (less than .4") at 100 yards, but for some reason, at 200 or 300 yards they will shoot groups small or smaller than they did at 100 yards. Most people believe this is related to shooting long high BC bullets in fast twist. Many times the nose of the bullet does not sit PERFECTLY centered in the bore, so they wobble a bit, and they settle down or "sleep" after they've gotten out of the bore and spin around the center of gravity and the nose gets right. When bullets are very long (high BC bullets), it takes longer for them to go to "sleep", and when they're over stabilized, it takes even longer. This translates on the target because the bullets are not quite perfectly whatever the bullet diameter is (e.g. .308). Since the bullet is wobbling, it's slightly wider; remember the bullet is spinning very fast, and the nose is not perfectly in line with the rear of the bullet. So the bullets are printing larger holes on paper than .308, and when you do it for 3-5 bullets it makes the groups look a fair bit larger. At a longer range, the bullets may have already gone to sleep, and so they're printing actual .308" groups. In this instance ,the bullets appear to be shooting "closer together" because they're not wobbling and are asleep. It should be mentioned that seeing this phenomenon is rather rare, and there's usually quite a few other explanations that are possible as well. Lots of times people just aren't concentrating as hard at closer distances, and have to be more in the zone at longer ranges, or they have parallax error at the closer range, etc. It is possible though that sometimes the cause of this is the bullet not being "asleep". Here is a video from Bryan Litz showing a graph of the nose of a bullet and how it can oscillate off of center until it goes to sleep. Watch how the spiral gets tighter and tigher to the center.



There's another thing that can happen when bullets are over stabilized. Bullets will "nose over" at the peak of the trajectory, so that they begin to fall with the nose down as the bullet is descending. If the bullet is overstabilized a fair bit, the gyroscopic stabilization will keep the bullet from "nosing over". This means that after the peak of the trajectory, the bullet begins to fly "nose up" (pointing towards the sky). This makes the bullet act like a wing (in a way), because there is a pressure difference caused by air moving faster over the top than the bottom of the bullet. This slight amount of lift will cause the bullet to hit slightly high and right (direction of rifle twist/rifling on bullet) from where the dope predicts it "should be". This means that your dope isn't right, which is a bad thing, and it can also have some other complications. It can cause the hits on target to look like they're slightly "key holing" because the bullets are flying nose up. Something else that's really bad can happen, and it has to do with when the bullet crosses into the trans sonic speeds. A supersonic shockwave starts at the very tip of the bullet, as the bullet goes trans sonic, the nose begins to pass through the shockwave and causes instability on even an "optimally stabilized" projectile. Normally the bullet will wobble and loose a bit of consistency in this region, but will correct it's flight and fly "true" after it passes into the subsonic range. If the bullet is flying with the nose up when it's starting to pass through the shockwave, the bottom/base of the bullet can get into the shockwave causes a LOT of instability. Lots of times when this happens, the bullet will NOT recover, and will just wobble and go completely unpredictable.

It's hard to tell which effects you'll get, and it's related to the bullet design, speeds, how over stabilized the bullet is, and ranges you're talking about. If you're shooting at closer ranges, you will never notice that the bullet is flying nose up after the peak of the trajectory and never get into the sub sonic range on paper. The reason being that it may not cause a drastic effect until after the peak of the trajectory, which is beyond the distances that most people shoot at. Hope this makes a bit more sense of some of the effects that occur with the bullets, and that they may or may not effect you.

[Tomcat088]

I can't explain it any better than the folks at Nosler so her goes from their #6 reloading guide: "The faster a bullet goes, the easier it is to stabilize. It doesn't spin any more than what the twist rate is in the barrel, it simply generates more rotations per second. For example, a bullet coming out of a 1-10" twist barrel will make one complete revolution for every ten inches it travels, even after it leaves the barrel, regardless of velocity. A bullet that is traveling at 3400 FPS will cover ten inches much more quickly than one traveling at 2400 FPS, and it can be said that the faster bullet is 'spinning faster', but it is simply cranking out more RPM and still not turning more than once every ten inches."

O.K. now part II

"A very common misconception is "overstability." Bullets do not become 'destabilized' or 'overstabilized' with faster twist rates, or greater velocities. They may not be quite as accurate as they were with slower twist rates or velocities, but they are stable. An unstable bullet may actually tip or tumble on its way to the target." Hope this helps.

Obviously the data is correct about the bullet never rotating faster than the twist of the barrel, but there can be some effects that do happen when they're spinning at quite a few more RPM's. If you think nothing happens, just watch bullets blow up before they get to the target because of those RPM's, I'd call that "over stabilized". When a bullet is moving so fast that the bullet design can't handle the twisting and air friction, it's "over stabilized". Other things happen as well, although we don't quite understand them all right now, or how pronounced the effect is. Either way, it's beyond the scope of what most people need or care to know.

This is being argued right now by some of the top bullet designers. Bryan Litz is well known to LR shooters and bullet designers. He's quite accomplished on the subject, and helps even when designing lathe turned solids, driving bands and gain twist projectiles. He's taken radar data of the bullets IN FLIGHT as well. He's helped design some of the software that ELR guys use on the tv shows where people are shooting .408 CT and .375 CT, that the rest of us are drooling over. He's come out with some of the most accurate G7 BC's for the major manufacturers; more accurate than the manufacturers themselves are able to get with their equipment or equations, they put those G1's to shame. That video I posted in my previous post comes directly from him. So while the bullets may not be "destabilized" so much that they tip end over end or deviate much from the path, that doesn't mean they're not "over stabilized". Nosler says that they may not be as accurate, they're hinting at something, but don't want to put themselves out there since it's not completely understood. There has to be a reason that they are not as accurate if they're spinning faster than the optimum twist, they just don't know.. There's a difference, and as I said, there's times when this may not have much of an effect until the bullet crosses into the trans sonic range and begins to pass into the shockwave. Either way, it is definitely possible to "over stabilize" a bullet, although people apparently want to use some different names or talk about the effect without any cause.

[Plain Ole Dan]

Wow, Thanks for all the info and the links (sorry I was somewhat brain dead and didn't even think about using the search function...blame it on the pain meds) And you guys made it simple enough for even me to understand.
Thanks again.

[Legion_Prime]

Damn Tomcat, it won't let me rep you but you absolutely deserve it. I even learned some stuff I didn't know about in there *LOL* Thanks!

[ReconLdr]

Piggy back on what Tomcat posted:

I completely agree. My rifle (SPS Tactical) shoots better groups at 200 and 300 yds than it does at 100 yds. At 100 yds my groups average .7 MOA and .5 MOA at 200-300 yds. This is with factory ammo. (FGMM 175gr BTHP)

I haven't shot paper beyond 300yds, but I would expect similar results.