.223 rate-of-twist

[Prof]

Still trying to decide on a new .223 bolt action rifle. One question I have is concerning the rate-of-twist (which I don't understand). Will a 1:9 twist rate be better for a 52-grain .223 bullet or would a 1:12 rate be better? And why? Thanks!

 

[Archie]

Usually, faster is better.

I have a .22-250 with a 1-14 twist. It shoots 52 grain bullets rather well, at 3700 f/s or so.

I tried some 69 HPBT bullets and cannot get them to stabilize. And I'm cranking them out at velocities to where they ought to be spinning fast enough to stabilize. But I can shoot a flat based 70 grain bullet very well.

If you are never going to shoot a bullet heavier than 52 grains, you'll be fine with a 1-12 twist. Since you're talking about a .223 Remington, you'll never be able to get them going as fast as my .22-250, and therefore, in equal twist barrels, yours will never spin as fast. So you have to have a faster twist to stabilize.

If you plan on shooting any of the longer, higher ballistic coefficient bullets, you'll want that faster twist.

Basic rule: A lot of overspin is better than a little underspin.

 

[Prof]

Archie: I only understood about half of what you wrote! Is the 1:9 a faster twist rate than the 1:12? I will shoot mostly 52-grain bullets but occasionally might want to shoot heavier ones. So, the 1:9 would be the better all-around choice for both? Right?

 

[Dave R]

I am not an expert, but what I have read is...

-1/7 is great for 62gr bullets, but can blow up lighter bullets.
-1/9 is best all-around compromize
-1/12 is great for lighter bullets, but won't stabilize the longer 62+grainers.

My .223 (SAR-3) is 1/8.5 or something wierd like that, and it WILL blow up the 45gr. Winchester varmit packs at 3600fps.

 

[wanderinwalker]

Prof,

Barrel twist is how far a bullet travels before completing 1 revolution, distance being measured in inches. A barrel with a 1:12 twist means the bullet completes 1-revolution per 12 inches of travel.

1:9 twist is faster than 1:12. If you are shooting bullets of 60 grains or less, the 1:12 is fine. The 1:9 will stabilize up to 75gr with enough speed. If you want to shoot the 77gr and heavier bullets, it gets interesting. For example, my AR-15 has a 1:8" twist. It will stabilize a standard 80gr .223 bullet very well. Some Highpower shooters use 90gr VLDs (Very Low Drag bullets) which require a 1:6.5 twist.

 

[Prof]

Okay, I think I've got it. Thanks, Wanderinwalker for that excellent explanation. And thanks Dave and Archie, too. As Dave put it, the 1:9 does look like the "best all-around compromise". I appreciate your help, gentlemen!

 

[Jim Watson]

First, you need to understand what you are asking about.
Rifling twist is specified as one turn in so many inches; 1 in 12 for example.
Fewer inches equals faster twist, the bullet is spinning faster at a given velocity and has greater gyroscopic stability.

The longer the bullet, the faster twist is required for stability. Longer, not necessarily heavier. Naturally a heavier bullet is longer than a lighter one IF the shape is the same. But a heavy roundnose can be shorter than a lighter spitzer and not need as fast a twist. Speer makes a 70 grain semi-spitzer bullet that will be stabilized by a 1-14 twist, for example.

Most .22 long rifles have a 1-16 twist for a 40 grain roundnose. Early .22 Hornets did, too, and that was ok for up to a 45 grain spitzer.

Early on nearly all .22 centerfires were standardized at 1-14 twists. This was great for the usual 52 grain boattail match bullet or a 55 grain spitzer varmint bullet and does well with a 60 grain flatbase in my .22-250. It is also enough for some rather blunt heavier bullets like that 70 grain Speer or a 63 grain Sierra.

The military tried a 1-14 twist for early M-16s. But that was not enough for stabilization of a 55 grain boattail in cold weather (dense air) so they went to a 1-12 twist.

But there was interest in shooting .22 centerfires at longer ranges for military and target applications. That called for long, heavy bullets. A 1-9 twist is right for the 68-69 grain bullets that were popular for a while. It would have been ok for the 62 grain half-steel core SS109/M855 military bullet, but the tracer version of that bullet is so long that it has to have a 1-7 twist. Match bullets are up to 75-80 grains and a .223 target rifle will usually have a twist of 1-7 to 1-8.

So if you are going to shoot 50-55 grain bullets exclusively, get the 1-12. It might well even let you shoot 60 grain bullets or the 64 grain Winchester game bullet.

If you ever want to shoot a 68 grain match bullet, get the 1-9. It will do ok with 52 grain bullets, "overstabilization" of bullets is rarely a problem if barrel and bullets are of good quality. But there is no such thing as a "little unstability." Too long a bullet for the twist simply will not hit anything at all, you will get "keyholes" of sideways bullets if you happen to hit the target.

If you want to shoot an 80 grain 600 yard match bullet, you will need an even faster twist, say 1-7.5 but there are not many standard factory rifles available with that twist; you would be buying a purpose-built match rifle for the competition and for the loads.

 

[Zak Smith]

If you ever want to shoot 75-77 or heavier bullets accurately, get a 1-in-8" twist.

-z

 

[Prof]

Wow, thanks, Jim! I'll spend some time tonight digesting all that information! I hadn't thought about different "types" of bullets. Will need to do some more research on that end of it.

 

[Badger Arms]

Seems like everybody's confusing twist rate with rotation. A twist rate only tells you the thread pitch, not the rotational velocity of the bullet. If you have a varmint rifle with a 24" Barrel, you're cranking the bullets out at higher velocity which will spin them faster. 1-in-9" twist on a 24" or 26" bolt gun will spin the bullet as fast as a 1-in-8" or 1-in-7" twist AR-15 with its shorter tube. Were it me, I'd get the 1-in-10" or 1-in-12" twist. The only exception would be if you intend to do a majority of your work at very long distances like 600-1000 yards. In this case, the 1-in-7" or 1-in-8" twist and VLD bullets in the 70-90gr range would be just the ticket.

One of the primary reasons the 22-250 has such a relatively slow twist is because it's intended for light bullets out of a longer barrel that are SCREAMING. IMHO, keeping the velocities between 3000 and 3400FPS is great for most .22 applications and that means bullets at or below the 62gr mark. On a bolt gun, you'd be just fine with 1-in-12" on a 24" barrel to stabilize these. It would then allow you to use the lighter bullets with greater flexibility.

I have no idea why anybody would shoot 22's at that range. You need a FAST bullet and high BC that 22's just can't meet. 6.5, .25, and 6mm projectiles do much better with the super-long VLD bullets in the wind.

 

[Zak Smith]

Are you saying 77gr bullets will stabilize reliably out of a 24" 1-in-9" barrel?

Remember that what we need spin (RPM) for is enough rotational inertia to keep the center of drag in front of the center of mass. As airspeed increases, the torque applied about the CoM by the CoD increases, too.

-z

 

[Badger Arms]

Zak, that's a tough question. 1-in-9" twist, 24" barrels are probably optimized for about a 63-65gr bullet or thereabouts (bullet shape notwithstanding). Rereading my post, I'd have to say I was recommending a 1-in-8" twist for 70gr bullets, so I don't really think the 1-in-9" would reliably cut it, but for you could probably pump the pressure up a bit more in a bolt gun and get decent results.

Of course this is all 'angels on the head of a pin' argument here. Each gun is different. Knowing the limitations of your gun takes empirical data. Anything we know up front might not play out when we start putting lead through paper. To answer your question, as a general rule 1-in-9" twist from a 24" barrel will probably stabilize a 77gr bullet, but it would't print good groups if you want to pin me down.

Figure 5-10% increase in velocity (from extra length and lack of a gas port) which imparts that much more rotational velocity and we can afford to push the established limits of bullet/shape vs. twist that are quoted for 20" barrels.

 

[Zak Smith]

Badger Arms,

My data is from my own guns, and what I see other people shooting in 3Gun:

1-in-9" twist 16-20" barrels seem to shoot 60-69gr bullets really well.

1-in-8" twist 18-20" barrels seem to shoot 75-77gr bullets really well.

I'm pretty sure the 75gr Hornady is too long for 1-in-9" twist based on mine and others' accuracy tests. The berger 73gr is supposed to stabilize in 1-in-9" but I haven't tried it.

-z

 

[artherd]

Remember that what we need spin (RPM) for is enough rotational inertia to keep the center of drag in front of the center of mass. As airspeed increases, the torque applied about the CoM by the CoD increases, too.

Not only that, the torque applied about the CoM by the CoD increases at V^2. I don't know offhand if the CoD movement caused by spin is V^2 or just V corrrelation. (or for that matter, V^3.) Though I probally should know this. Zak?

 

[Zak Smith]

Well, let's see. I ought to break out my copy of Modern External Ballistics..., by McCoy which would have the definitive study, however.. since it's not handy right now. I think this is correct:

drag(v) = Cd(v) * i * r^2 * v^2 * p

Cd = coefficient of drag, function of velocity (roughly 1.146 - 1.047E-4 * v from 1500 to 3600fps).
r is radius of bullet
v is velocity
p is air density
i scales the observed drag to standard drag, i = m/(d^2*BC)

So drag is roughly proportional to v^2

Moulton gives a criteria for stability when

transverse (tumbling) angular acceleration < 1/4 * w^2 * (Il/It)^2

where w is longitudinal angular velocity, note w = 2*pi*v/tw

This would imply that the spin stabilization is proportional to v^2 also.

Thus the only "v" term that doesn't drop out is Cd(v) which is a function of v.

Note that the rough estimation of the Greenhill equation does not depend on velocity either.

-z

 

[Badger Arms]

Zak:

Remember, I was speaking about a 24" barrel, not an 18-20" barrel. I was also speaking about a solid-bore barrel, not a vented autoloader.

 

[Archie]

Badger...

you'll notice in my earlier posting I made reference to the difference between twist and rotational velocity.

One of the complicating factors is that with increased velocity of projectile (thereby increasing rotational velocity) comes increased wind resistance. This increased wind resistance makes for greater instability; that is, the bullet is more likely to yaw or vary from true (straight) flight. That is indeed what happened to me with the Sierra 69 grain HPBT match bullets. They just won't stabilize in a 1-14 twist .22-250. And you may take my word for it I was getting all the rotational velocity the law would allow.

When I say they would not stabilize, allow me to present the following information. With a 55 grain bullet and fairly top end loads, I was getting groups of .7XX inches at one hundred yards. With the Sierra 52 grain match bullet, I would get groups of .4XX inches. With the 69 grain bullets, I would get an average of one hit somewhere on the target backing! Moving back to 50 yards, I found a few more hits (3.5 of 5 shots), about one in five in profile - perfect keyhole.

Moving to the Speer 70 flat base round nose, the groups again are in the sub 1" area and awaiting further development. The 70 flat base bullet is something like a tenth of an inch shorter. (I dont' have my notes, I'm working from memory here, forgive me, but the information is pretty close and the results are genuine.)

As our colleage Mr Watson said

..."overstabilization" of bullets is rarely a problem if barrel and bullets are of good quality. But there is no such thing as a "little unstability."

In my own words, a lot of overspin is better than a little underspin.

On overspin. The late P. O. Ackley reported blowing up some bullets with an experimental .219 Donaldson Wasp. He built a rifle with a 1-5 (yup, one turn in five inches) twist. He shot some fairly fast loads with a 40 grain Hornet bullet and couldn't find any holes in the target. After much research, he found they were overspun and coming apart at about 5-10 yards out of the muzzle. With a slightly heavier, tougher bullet, he found the bullets were bulging at the 'equator', so to speak, to about 30 caliber.
So yes, there is a possibility of too much spin, but it's really hard to achieve (1-5 twist barrel?).

 

[Zak Smith]

Yes, I know.

I am not convinced that rate of twist required for stabilization is a function of velocity.

However, it is a function of atmospheric conditions. 75-77gr from a 1-in-9" might stabilize on a hot day at 5000' altitude, but it's not going to in the winter in Wisconsin.

-z

 

[Zak Smith]

Archie mentions that a 70gr flat-base round nose bullet shot well, while the 69gr HPBT didn't stabilize.

This is because the lontigudinal rotational moment of inertia (ie, around it's axis) is larger for the FBRN than the HPBT because it has more material further from the axis. Another contributing factor is that the 70gr FBRN is shorter than the HPBT and so the center of drag is closer to the center of mass, reducing the "lever arm" distance which applies transverse torque.

-z

 

[Jim Watson]

I read somewhere or another, maybe one of the ballistics texts will spell it out, that bullet stability was proportional to the fourth root of velocity. Same bullet, twist, etc.

 

[Prof]

Let's get this conversation back to English for a minute . If I'm following this conversation (and I'm not getting most of it), if I have a Savage 12FV in .223 with a 26" heavy barrel and I'm shooting from 100 to 300 yards, the 1:9 twist of that barrel would shoot 68 or 69 grain bullets best but would also handle the lighter 52, 55, etc. grain bullets as well. Correct? (By the way, I'm really impressed with the knowledge some of our THR members have!!! ). Thanks!

 

[Zak Smith]

Yes.

edit: as long as the 50gr bullets don't explode due to high RPM.

-z

 

[Prof]

Thanks, Zak! I'm going to print this thread out and keep it. Maybe I'll eventually understand all this stuff!

 

[Badger Arms]

Zak:

I understand what you are saying about rotational velocity and wind resistance. I don't think we disagree on that matter, only that you feel that velocity is LESS of a factor than I believe. We can get along on that factor.

One thing that hasn't been mentioned (that I've seen at least) is the effect that twist rate and velocity have on barrel life. Let's say you're pushing 3500fps with your 50gr loads and have decided on a 1-in-8" twist barrel because you want to be able to shoot 77gr bullets. I'd imagine that your throat and bore erosion would be nigh onto severe.

My suggestion for the Prof would be do decide what the majority of the shooting you plan on doing is going to be done with in terms of load and bullet weight. Find the OPTIMUM twist rate for that load and go with it. You might have to live with inflexibility in terms of maximum and minimum bullet weight, but you'll be happiest with your chosen load in the long road. My vote, I'd shoot 50-55gr loads all the time and maybe have a 62gr longer range jobbie and maybe a lighter load for the smaller pests. If the stated range is 100-300 yards, you really don't need anything faster than 1-in-10" twist because you aren't shooting at the longer distances where a heavier bullet would really give you the edge.

Bottom line: 1-in-10" is the proper answer!

 

[Zak Smith]

Well, it's not quite a feeling since I have math on my side. I think the required twist to stabilize a bullet won't change as MV changes, as long as we're operating in the 2000-3600fps range, at least.

I do agree with your advice, however. No sense in a fast twist if you're shooting 55gr bullets.