Question on Twist Rate

[Brian in Montana]

I understand that heavier-for-caliber bullets tend to be longer and require a faster twist to stabilize. Does that necessarily mean that a fast twist wouldn't stabilize a lighter bullet well?

 

[MThuntr]

I have zero evidence or experience with this but I'm read a bullet spinning too fast is unstable (something like 1 degree off axis which isn't probably noticed at 100 yards), causes excess spindrift at distance, and/or can separate at the muzzle if the jacket is thin.

 

[Brian in Montana]

I have zero evidence or experience with this but I'm read a bullet spinning too fast is unstable (something like 1 degree off axis which isn't probably noticed at 100 yards), causes excess spindrift at distance, and/or can separate at the muzzle if the jacket is thin.

This does make sense to me. It seems there would be something of an optimal spin to stabilize a given bullet, not heavy and light, but I don't know a lot about that.

 

[VikingsGuy]

I am no expert, but my understanding is that this is a “Goldilocks” issue. Not to fast, not to slow, you want it just right

 

[brockel]

I ran a 8” twist 220 Swift Ackley 4400fps with a 40 grain nosler ballistic tip and never had accuracy issues.

 

[Deleted member 16014]

I ran a 8” twist 220 Swift Ackley 4400fps with a 40 grain nosler ballistic tip and never had accuracy issues.

Wow, that's screaming. I like it.

 

[Pelican]

This does make sense to me. It seems there would be something of an optimal spin to stabilize a given bullet, not heavy and light, but I don't know a lot about that.

Greenhill’s formula can be used to supposedly calculate the optimal twist rate for a bullet. Over stabilization doesn’t seem to be a much of a problem in the real world though.

 

[brockel]

Wow, that's screaming. I like it.

Barrel was toasted pretty quick

 

[memtb]

If a bullet spins too fast, I believe that the term is called “yaw”. The bullet can actually be de-stabilized by “overspinning”, causing a loss of accuracy! memtb

 

[antelopedundee]

I have zero evidence or experience with this but I'm read a bullet spinning too fast is unstable (something like 1 degree off axis which isn't probably noticed at 100 yards), causes excess spindrift at distance, and/or can separate at the muzzle if the jacket is thin.

Wouldn't that be akin to a curve ball vs a knuckle ball sort of thing? Wind direction can also either oppose it or reinforce it.
Poorly constructed bullets can spin themselves apart.

 

[antelopedundee]

I understand that heavier-for-caliber bullets tend to be longer and require a faster twist to stabilize. Does that necessarily mean that a fast twist wouldn't stabilize a lighter bullet well?

As long as the holes in the paper are round instead of oval or oblong shape [ it's called keyholing] I would't worry too much about it. Generally a slow twist is not preferred for heavier bullets.

 

[cahunter805]

It’s better to be a bit over twisted than under. In most calibers you can still shoot lighter weight bullets with a faster twist and be just fine unless you went drastically faster twist.
Also be aware that just because a bullet isn’t keyholing on paper doesn’t mean it’s completely stable. You can have a bullet that shoots good at 100 but get to say 400-500 and it won’t group at all. Over the distance it looses velocity and stability and starts to yaw.
I tend to always twist my rifles a bit faster than needed. There are always new heavier bullets being developed these days also!

 

[BrentD]

Greenhill’s formula can be used to supposedly calculate the optimal twist rate for a bullet. Over stabilization doesn’t seem to be a much of a problem in the real world though.

the Greenhill formula, and 4 or 5 other similar formulas, are used to predict the slowest reasonable twist rate for a given bullet length, not the optimum. In either case, the best or minimum twist rate is somewhat subjective in that what may be reasonably stable at 200 yds may be perfectly unstable at 1000 yds however.

 

[ImBillT]

Too many RPM can contribute to mid-air blowups. Short range benchrester like the minimum twist. They say that too many RPM’s magnifies imperfections in the bullets. It seems like tighter and tighter twists are giving good accuracy these days, likely due to better bullets.

Generally, too much twist is not something to worry about. For a while(and possibly still) Berger had the thinnest jackets on the market. This contributed to their consistency. They had to start making thicker jackets because of mid-air blowups. The VLD-H is the original jacket, and the VLD Target is the thicker jacket. I asked about how much twist would be too much for the hunting jackets, and they told me that around 400,000rpm was where things started getting iffy. You can calculate that from your twist and muzzle velocity. They said the issue was actually that friction in the barrel would melt the core, and that once the core melted, the bullet would blowup somewhere down range. Factors contributing includes twist, barrel length, bore finish and condition, rifling depth, shape and sharpness, and barrel temp. Long shot strings in a long, cut rifled barrel with a worn throat would result in blowups very close to 400,000rpm, while infrequent shots through a short, button rifled barrel with a fresh throat might allow well in excess of 400,000rpm before blowups became an issue.

 

[sbhooper]

Having a barrel twisted for long bullets, is way more apt to shoot short bullets well, than the reverse. With long bullets, there is no free lunch.

 

[std7mag]

I hadn't heard of Bergers "blowing up".
I had read that they went to a thicker jacket due to fliers at the end of a string, caused by the hot chamber/barrel causing core/jacket separation.
"Blowup" is usually associated to the "bad old days" of bullet development.

 

[HandgunHunter]

You can shoot lighter bullets in a faster twist.
Only in the small screamer calibers should you have issues with bullets blowing up.
Normally if you go with a faster twist you will also have it cut for a longer throat for the longer bullets. This may cause you to have excessive bullet jump with the light ones.

 

[ShadowFast1]

As previously mentioned some bullets are prone to coming apart if spun to fast. A good quality modern bullet shouldn't have much of a problem with a fast twist.

There is a group of people who have come to like faster than normal for caliber twist rates as they've found the bullet stays stable better during the trans-sonic/sub-sonic transition. Think .30 caliber with 1:7 twist instead of a 1:10. This is really only for folks who are pushing the envelope on long range shooting.

Then there's the accuracy aspect. I think there are optimum twist rates for cartridges. An example is my dad's .300 Ham'r. It comes with a 1:15 twist but that's because the case isn't designed to handle bullets over 150 gr. Unfortunately when some cartridges were designed they didn't spec barrels with fast twist rates because they didn't foresee people firing heavy for caliber bullets in them. E.G. .243's 1:10 vs 6 Creedmoor's 1:7.5. A faster twist can also exaggerate bad bullet construction or poor bullet concentricity.

All of this to say that a fast twist will stabilize the lighter bullets but they may not be as accurate compared to an optimized twist rate.

 

[ImBillT]

I hadn't heard of Bergers "blowing up".
I had read that they went to a thicker jacket due to fliers at the end of a string, caused by the hot chamber/barrel causing core/jacket separation.
"Blowup" is usually associated to the "bad old days" of bullet development.

Jacket core separation is a blowup. Some F-Class guys were shooting 10,X, 10,10,X,miss, X, miss etc. and determined that the bullets were coming apart before hitting the target. Berger tracked it down to the cores melting. It was mostly relegated to really big cartridges shooting heavy bullets in tight twisted barrels. They decided to thicken the jackets slightly, but had recently begun recommending VLD’s for hunting. When they tested the thicker jackets, they didn’t always get good expansion, so instead of thickening all the jackets, they split into Hunting and Target lines. The Hunting is the original jacket thickness. 300,000 RPM was about the lowest RPM they were having issues at, and that’s pretty fast. I said 400,000 RPM in an earlier post, but it was 300,000 RPM. In an 8-twist barrel, that’s 3333fps, in a 7-twist that’s 2915fps. There aren’t all that many cartridges that can move bullets requiring a 7-twist over 2915fps, but you can see where putting too much twist into a barrel COULD result in a blowup if you were shooting light bullets through a tight stool. Still, most bullets will handle it just fine. Generally, too much twist it better than not enough.

 

[std7mag]

We appearantly have differing definitions of "blowing up"!

To me there is a major difference between blowing up, and core jacket separation. Especially if the core is still in the jacket.