Not Understanding Twist Rates

[TargetMagnum]

I am looking buying a Remington X-100 .223 but I am not understanding the twist rates. I would like to do a lot of target shooing at many different distances. When I was in the Marine Corps we shot the M-14 and the M-16. I was in when they were phasing in the M-16 and I really liked it. I remember shooting it at the 500 yd mark and I don't remember it having any trouble reaching the target at that distance.
Anyway my question is what is the best twist for an all around shooting at several different distances? Also can anyone explain why the twists are different?
Thanks
TM

 

[M700]

Twist rates are different to stabilize different bullets. Longer, heavier bullets require a faster twist rate to stabilize 'em.

Back when the .223 was young, everyone shot light 50 - 55 grain bullets and a slow twist worked well. Many rifles today still come with a slow twist.

In the mid 1980's the military adopted the M-16A2 with a much faster/tighter twist rate to stabilize new heavier bullets. Wasn't long after that people began to figure out that really long, heavy .22 bullets fired from a fast twist barrel, worked real well at longer ranges...

We created our own monster. Now we've got people who want slow twist barrels to shoot 40 grain varmint-zapper bullets and also people who want crazy fast twist rates to stabilize 80 grain bullets.

Go figure. It's all fun. Just figure out which bullet you want to shoot, and make sure your barrel will stabilize it. For example, a 1/9 twist is pretty common, and will stabilize a 69 grain bullet. It also works well for 40 - 55 grain varmint bullets. It's not so hot with 80 grainers...

Regards, Guy

 

[Twud]

In very basic terms the heavier the bullet the faster twist you need to stabilize it.



http://sst.benchrest.com/shilentwist.html

 

[lazylabs]

Speed also plays a part in this. A given bullet will stabilize at a given RPM. The heavier for caliber the bullet the more RPM's it takes. The RPM is important to consider when your at either end of the speed scale a 80 grain bullet in a 222 rem will require a faster twist barrel than the same 80 gr bullet in a 223 WSSM or 220 swift. If you are decided on a case then pick the heaviest bullet you want to shoot and pass that combo to the smith or barrel manufacturer your ordering from and they will get the gun set to match your idea. It's safer to error on the side of faster twist but shooting 55's in a 8 twist barrel probably won't get you the best possible accuracy. Some will say it slows them down,never seen proof of that..PERSONALLY) It only takes a minute to ask a couple smiths, barrel makers and call to a couple of the bullet guys and you can get a few opinions. If you have a prefered barrel maker they may only offer a few twist combo's anyway.

 

[lazylabs]

I mis-spoke about the heavier bullet taking more RPM's to stabilize.... I ment a faster twist to achieve the same RPM's due to decreased velocity.

 

[mikecr]

Stability does not follow RPMs.
Required twist is in inches per turn. This is displacement per turn.
NOT turns per time.

 

[TargetMagnum]

Now I am really confused.
Time vs. Revolutions
Maybe I better leave this to the Pro's
TM

 

[mikecr]

Well bullets are spin stabilized, so there is a relationship with spin RATE. But in flight, IMHO, the appropriate RATE which passes more tests, is turns per displacement.
With this approach, it makes sense that gyroscopic stability goes up, -down range, regardless of RPMs. A bullet's turn per displacement is increasing as it travels, slowing more in velocity than spin. So stability may go through the roof downrange, possibly causing 'overstabilization'.

There is more to it of coarse. Much of it depends on drag.
But assuming that stability follows RPMs will fail tests pretty quick. For example, you will not stabilize a 140 Berger with a 1:12tw barrel, no matter the velocity. It will make no difference if fired at 2,000fps or 5,000fps. None. Only when a bullet is right on the edge of stable, will a big adjustment in velocity possibly help. A drag change, and air density affects this more..

 

[lazylabs]

So your saying that a 10 twist barrel will stabilize 220 Gr sierra's in a 300 win so it will also stabilize them in a 300 whisper? If rpm doesn't matter this would be true....

 

[mikecr]

Actually it looks to me like 10tw will stabilize a 220smk up to 960fps at 5,000'ASL, and 75deg.
10tw will stabilize this bullet again @ 1200fps and above under the same conditions.

It doesn't like picking up the shock wave in dense air right at mach1 with such a marginal TWIST rate. Drag peaks nearest mach1 and goes steadily downward from there. So an increase in twist rate would help in this region for sure, and that's about all you can do other than shooting at higher altitude & temps?
You could go with an 8tw and be fine above. And this would cause an increase in RPMs, sure. But it would just fail again at sea level and 50deg.
Think of it as displacing 10" of heavy air per turn,to overcome this drag and keep from overturning).
Stability is tied heavily to bullet drag, length, and it's center of gravity w/resp to it's in-flight center of pressure. 3 things contributing nothing to, or affecting, centrifugal force. If you put most of the lead in the front of the bullet rather than rearward, you would need NO twist at all. ZERO RPMs.
This bullet would stay nose first, following trajectory, just like a dart.

I'm not a math wiz, and couldn't baffle anyone with a detailed analysis here. I rely on software and put 2 & 2 together to come to this side of the fence. It's fun to think about and try to understand.
Here's another extreme example to show my point:
A 107smk out of a 10tw barrel whacks paper sideways at 25yds here just off a sunny beach,80deg). My software shows that this bullet will self destruct at 305Krpms/4243fps and no where near stable. In fact, if I could push it to 6000fps,432Krpms), and it held together, it would still tumble immediately.

Yet it stabilizes from a 9tw @ 2850fps,228Krpms).

It's not RPMs.

 

[CPorter]

Mike,
What software are you using? My software and most of what I've read says the stability IS related to rpm. The Sg,stability coefficient) goes up with velocity in the same twist rate and somewhere between Sg = 1.25 and 1.5 the bullet will stabilize.

I will agree that bullet construction does have an effect as well. If the Cg of the bullet could be moved well into the nose area then the lift caused by air flow over the nose would have less or no effect.

 

[mikecr]

I didn't mean to imply that stability isn't related to RPM. What I'm trying to suggest is that stability does not directly follow RPM.
That bullet makers could not designate RPMs needed to stabilize.. It would be a complete disaster if they tried this.

I use my own compilation of Davis/McCoy code validated against JBMs MPM, QuickDesign, and Henry Childs testing of various bullets & conditions,Some of which was surprising and not easily predicted).

I'm am aware of a spreadsheet out there that implies RPMs needed for stability. It's completely wrong. I've seen a bunch of rules of thumb, which also will fail tests. This is because they only define a narrow window accurately. For instance, I have a formula given to me which defines stability for bullets by weight, twist, and RPM's. When I dug into it, I found that it's source simply defined a common twist chart 'enhanced' with normal velocities/RPMs for these calibers,in competition). It could fool many, it could screw many.
I could define stability for my 139Lapuas, and put it out there as a 'new rule of thumb for stability'. It would SEEM to work for closely related bullets, velocities, and conditions. But that would do little more than mask reality.
So watch out for these.

Pick up 'Modern external ballistics' by RL McCoy. This would start anyone down the same path I'm following. And I'm only in it for my personal gain. No more or less.
But It's difficult to watch assumptions grow into difficult to dispel -PsuedoFacts.
Before you know it, the world might have to be proven round again.

 

[halfmile]

Sierra would not or could not give me any RPM info on their bullets but apparently they do have data suggesting different BC's for different velocities.

Read more:

http://yarchive.net/gun/ammo/bullet_spin_rates.html

HM