Barrel twist question for handguns

[DMY]

Reading, but not wanting to hijack a thread from another poster, I wonder whether there are recommended rifling twist rates for handguns and certain bullets weighs and whether it is dependent on the barrel length and bullet speed. I know there are certainly reccommended bullet weighs for certain rifle twist rates for AR-15s. Anyone know of emperical data or a good article on the subject. Judging from some of the responses in the other post, I presume some may suggest a 1:18 might be common for the old PPC revolvers with bull barrels. I believe they were usually 6 inch barrels shooting 148 gr wadcutters, typically low-ish velocity, but that is only my guess.

[cw308]

Good question , I would guess there is a formula on twists for proper function . 1 in 12 just think what would happen if that was 1 in 6 .

[Radny97]

Copying from another forum. Credit Iowegan:


Twist rates in rifles are designed to keep a bullet stable several hundred yards downrange and are geared for specific bullet lengths. As you may know, a bullet's spin rate will decay from air friction as the bullet travels so at some magic point downrange, the bullet will no longer be spinning fast enough to maintain stability and will begin to yaw, then soon after it will begin to tumble. Accuracy (not counting bullet drop from gravity) will maintain quite well until the bullet begins to yaw. With most centerfire rifles, the stability/accuracy distance can easily be 300~500 yards ... even farther with some bullet and twist rate combinations…

{Edit: Please read the board policy on posting copyrighted material (and yes, all forum content are considered copyrighted). A few sentences are OK. Otherwise, you have to get permission from the original poster and indicate in the post that you have that permission. Much easier is just to post a link to the other forum's post, so they get credit for eyes on their site when folks read it.}

[Jim Watson]

An optimum twist rate would result in a handgun twisting in your hand way too much to control accuracy.

Hmm. Standard 18-20 twist .44 Magnum does not torque enough to hurt accuracy at 200 metres in my experience. Some old guy name of Keith thought it was ok a good deal farther than that.

And what is "optimum?" The Miller stability factor for about any pistol is far above what would be considered suitable for a rifle. That .44's stability is 4.48 where 1.5 is considered adequate.

At the extremes, Jerry Keefer was making 12 twist .38 and .45 barrels for 750 fps target loads, while Schuemann makes 32 twist .38 Super barrels for 1400 fps raceguns.

[Unclenick]

Originally Posted by Radny97

… As you may know, a bullet's spin rate will decay from air friction as the bullet travels so at some magic point downrange, the bullet will no longer be spinning fast enough to maintain stability and will begin to yaw, then soon after it will begin to tumble. Accuracy (not counting bullet drop from gravity) will maintain quite well until the bullet begins to yaw. With most centerfire rifles, the stability/accuracy distance can easily be 300~500 yards ... even farther with some bullet and twist rate combinations…

I hate to tell you, but that's complete nonsense. The rate of bullet rotation falls off due to friction with air from its rotational surface speed, only, which is much less drag than a bullet's forward velocity causes, so rotation drops off much more gradually than forward velocity does. Without barrel rifling, the two motions, forward and rotating, are completely independent of one another and are not subject to the same forces. Rifling engraving marks are typically smaller than the air boundary layer over the bullet, so they don't affect either motion appreciably. In other words, stability normally increases with range.

There are some oddball things that can happen to some bullets that may create the illusion of the posted description. The 168 grain .308" Sierra MatchKing, for example, is famous for becoming unstable in the transonic velocity region (about 950-1400 fps at sea level). For most 308 Winchester rifles, this begins just beyond 700 yards. Bryan Litz has explained this is the result of a dynamic instability caused by the bullet profile overcorrecting for overturning forces. But if you shoot a design like the 175 grain Sierra MatchKing that does not have that transonic instability, they fly right through the transonic range with no problem and stay pointed forward for as far as you can fire them. If it weren't so, snipers would be without working equipment when really long range shots came up.

Here is an example. Note how slowly the RPM drops off as the bullet goes downrange and how the stability factor actually increases with distance because the frontal drag is dropping so much faster:



When a bullet clears the muzzle it undergoes initial yaw as it changes its spin axis from sharing the bore axis to flying at the yaw of repose (typically in the tenths of an moa off straight ahead, initially) which is where precession gets into equilibrium with overturning forces. It takes that bullet a couple hundred yards to lose 80% of that yaw (going to sleep).

Rifle bullets are usually fired so they start out with a stability factor that is not too high to avoid eccentric spin (wobble in flight), core stripping in the barrel, or flying apart on the way downrange. However, at pistol velocities you will have less trouble with any of that, so a pistol can often be given a much higher stability factor and a proportionally faster twist than they strictly require just to avoid tumbling, and not suffer problems from it.

That high stability factor seems to have an accuracy advantage. It's an illustration of the fact "over-stabilization" isn't, practically speaking, a real thing. It is actually the problems I mentioned for rifle bullets that you are trying to avoid by not spinning them too fast. When stability factor is high, the bullet's trajectory is harder to perturb and it seems to be more immune to initial yaw, muzzle blast asymmetry caused by very slightly imperfect crowns, etc.

There was an article twenty years or so back about twist rate for the .32 S&W Long HB LWC's used in International Pistol centerfire guns. The standard rifling pitch 18¾". It should give the bullet a stability factor of about 2.5, which is plenty. IIRC, the author got 16", 14", 12", and 10" twist barrels made. As the author went down in pitch length, groups got tighter and tighter until he got to 10", where they started to open up again, possibly due to the hoolow base skirts of the bullets starting to spread from the centrifugal effect on them. But at 12", there is a stability factor of almost 6. In theory, totally unnecessary, but it seemed to help with that low velocity round.

[RC20]

Or we can look in Sierra and Horandy (and any other manual that shows the gun used) and see what twist they have.

A few outliers that were historical and not so good but those are obvious.

Most if not all modern version corrected that if it was off.

[Unclenick]

Well, the example of the 32 wadcutter is one where the twist rate is historical, as it is for a lot of old chamberings. It may well have been best for the round nose bullets originally loaded in it, but it may not do best with longer designs. At the same time, I wouldn't put on a barrel with that 12" twist and expect other bullet shapes at other velocities to like it equally. That is, I don't think the experiment proved a 12” twist is necessarily going to provide the tightest groups with anything other than the wadcutters..

[Radny97]

Sorry about the over ambitious quote from another forum. I was unaware of the copyright issues. I’ll paste the link below. To be clear, I’m not saying that the poster on this other forum is 100% correct or not. I’m not well informed enough on this issue to be sure on that. But i thought the information he provided on this issue was helpful to the discussion.


http://rugerforum.net/reloading/1266.../topics/126680

[Unclenick]

The basic reason he's off is he is assuming bullet rotation decays at the same rate as velocity. He says:

Originally Posted by Iowegan

Yes! After the bullet exits the muzzle, the bullet's spin rate will decay at the same percent rate as velocity.

That's patently untrue. If the barrel length went all the way to the target then it would be true (if friction didn't stop the bullet in the bore on the way) because the rifling would then keep rotation rate tied to forward velocity. But, obviously, that's not what we shoot.

If you were to fix a microscopically small anemometer to the surface of the bullet, it would measure wind speed that was a vector product of the forward velocity and the rotational surface speed of the bullet. Only the vector component due to the rotational surface speed of the bullet has friction slowing the bullet's rotation. Moreover, because the boundary layer of air prevents the rifling from mattering to this, there is no frontal surface area crashing into the wind as the cross-sectional area of the bullet does going forward through the air, so the bullet rotation is slowed by what is called laminar fluid surface friction, alone. Where frontal area drag increases as the square of velocity times the drag coefficient of the shape at each velocity, the laminar drag is merely proportional to surface velocity. So we need to see how these velocities compare.

Taking the poster's example,

Originally Posted by Iowegan

Example: a 223 Rem barrel has a 1:8 twist rate and drives a bullet to 2880 fps. 12/8=1.5; 2880 x 1.5=4320; 60 x 4320 =259,200 rpm.

But now let's do what he left out: calculate the surface speed of rotation whose friction acts to slow the spin of the bullet. Forward velocity is 2880 fps, so let's see how many fps the rotational component at the bullet surface has:

Bullet diameter = .224 in., so the circumference of the bullet is pi ×.224=0.704 in.

The bullet surface rotates through 0.704 in./turn

0.704 in./turn / 12 in./ft = 0.0587 ft./turn

The rifling pitch in 8 in/turn. or 0.667 ft/turn.

If we call feet of circumference ftc and feet of bullet forward travel ftd, then:

0.0587 ftc/turn / 0.667 ftd/turn = 0.0880 ftc/ftd, meaning 0.08796 feet of bullet surface rotation for each foot of bullet travel in the bore. Now we just multiply velocity at the muzzle by that ratio to get bullet surface speed of rotation:

2880 fps × 0.0880 ft/ft = 253 fps of surface rotational speed.

But there's more: remember the bullet presents cross-sectional area to the headwind its velocity creates, but in the rotation the rifling marks are not tall enough to present any sort of significant paddle area to the rotation, so bullet slowing forces are even greater, proportionally than just 11.4:1 (2880/253). For example:

An SS109 bullet fired at 2880 fps loses just about exactly 1 fps for each foot of travel right after leaving the muzzle.
Rotation loses about 0.042 ft/s of rotation for each foot of forward travel near the muzzle, or about 1/24 the rate of surface speed loss that the bullet has in forward speed loss.

Ballistician Geoffrey Kolbe has a formula for rotation loss that I've used to estimate the above numbers. You take the starting rotation rate in any unit you like, calling it N. You then need the time of flight, tof, from the ballistics program of your choice. To find rate of rotation, Ntof, at the end of that time of flight in the same units:

Where d = bullet diameter in inches

Ntof = N × exp(-0.035 × tof / d)

If you are not familiar with the exp() notation, it just means the natural logarithm base, e, raised to the power inside the parenthesis. I used it here because we can't make superscripts or subscripts in text on the board. Use the e^x function on the Microsoft calculator, where x is what's inside the parentheses. Excel accepts the exp() notation.

The other thing that post says that I find baseless is the assertion that a "correct" rotation would necessarily introduce too much torque to handle. He must think an ideal twist is very fast, but I don't know any reason to assume that automatically. If, like the .32 wadcutter example I gave, the best accuracy was produced by a very fast twist, it might be so, but as I said, I think that situation is peculiar to that bullet and the velocity range it is normally loaded to. I've had the standard 20" twist in a .44 Magnum group 240 grain soft points into under an inch at 50 yards from my Redhawk. I think that's down around the limit of my ability to hold a handgun shooting off bags, so I don't see how much more optimal it could get, and torque wasn't bothering me there.

Overall, Iowegun just seems to have a fictional sense of how exterior ballistics work.

[hounddawg]

Schueman Barrels has some good info on pistol twist rates

http://www.schuemann.com/Portals/0/D...Twist_Rate.pdf

[Radny97]

Very well deconstructed unclenick. Thank you! That helped me understand the physics much better.

[Unclenick]

Glad to help.

Schueman is mostly right. He's confused the coning motion caused by precession during recovery from initial yaw with precession itself and fails to mention that the circling and the nutation gradually damp out when the stability factor is greater than 1 and a couple of other nit picking details, but he's got the basic influences and reasoning behind picking an optimal twist right.

[Don Fischer]

A good thing about twist is that most factory rifles come with twist's that will stabilize Most the bullet's available in that cartridge! Getting faster twist's in after market barrels will let you up the weight of the bullet's you can use but generally you are better off with different cartridge. I can't understand those getting fast twist 223's so they can shoot the heavy weight. You shoot a 70gr bullet in your 223 and you can get more velocity and range shooting something like a 75gr in a 243. Plus you can got up in weight to 100 to 105 gr from there in the 243. If you want to shoot a 120gr bullet in a 243, if it was made, you'd be better off going to a 25-06! Things are all nice to talk about but really make no sense to me!

[Unclenick]

Don,

The fast twist .223's come from two places: One is that the National Matches were originally organized to promote civilian rifle practice with current military style weapons so military draftees would have shooting skills already established. For that reason, they also compete against the military marksmanship units at the National Matches, and those units have military weapons modified by their armorers to work well with longer, higher BC bullets, like the 80-grain MatchKing. If civilians competing in the service rifle phases of the National Matches didn't copy the faster twists, they'd be at a big disadvantage against the marksmanship unit team members.

The second place it comes from is the military service weapons. The M-16 and its M-4 variant have a 7" twists these days. The reason is to be able to stabilize the long-and-light-for-weight tracers, like M856, associated with M855 or other SS109-like ball ammunition, and to do so even in dense atmosphere as occurs in deep sub-zero weather and below sea level conditions when more spin is needed to keep a bullet stable. Bullet length has more effect on twist requirements than weight does, and these tracers are about 30% longer than the ball bullets they are supposed to fly with.

The bottom line is, .223 fast twists come from the military and the civilian market adopted them, same as it adopted the 10" twist for .30-06, even though that fast twist was originally for 200 grain 30-40 Krag bullets.

If you go to a modern commercial rifle, I am with you. For long range, I would choose the 6.5 Creedmore over the 280 Remington because of the standard faster twist and longer throat meant to accommodate long bullets sticking out further and do so without taking up powder space.

[Stats Shooter]

One thing I will add to this conversation.....Unclenick stated that length has more to do with stability/instability than weight, which it does, but we use weight as a proxy because more weight = more length since the bullet cannot get fatter and still shoot in the same bore.

But, it isn't as big of a deal in larger bores like pistols or big bore rifles as it is in the .223. This is because volume increases with the cube . So adding 50% more weight doesn't add the same ratio of length as it would in a .223. I.e going from a 50 gr 223 to a 75 gr 223 will add more relative length than going from a 100-150 gr 30 call bullet (assuming identical bullet construction and shape).

And of course many pistols are much larger than .22 call so it's easy to find a universal twist because the relative change in length for a given weight is small.

[Unclenick]

Hmmm, if I understand correctly, I think you're having brain gas moment. The proportional increases will be the same if the shape is the same. It is added grains of weight that don't make a proportional weight difference in the different diameters. If I take a cylinder and double its length, the weight doubles because now I have the equivalent of two of those cylinders. That will be true regardless of what diameter or length the cylinder is to begin with.

The problem we get into with weight as a surrogate for the weight/length combination these days is it was originally premised on the idea all bullets of the same weight would have about the same construction and similar shapes and would, therefore, maintain that length proportionality. But when copper solids came out, being less dense than lead, that messed this up. When VLD shapes came out and were longer for their weight than the old conventional boattail shapes, this likewise messed with the proportionality enough that some twist rates ceased to work out. That weight-based twist approximating went through its first cycle of revision after the turn from the 19th to the 20th 20th century as shooters gradually went from predominantly round nose bullets to the spitzer and boattail shapes of WWI and II, which were longer for their weight than the RN designs.

[Stats Shooter]

is added grains of weight that don't make a proportional weight difference in the different diameters.

That's what I was trying to say and misspoke about the proportionate increase. I should have said nominal....I.e. a 50 grain increase from a .223 bullet would yield more nominal length than a 50 grain increase in a 30 cal. all else equal.

[Unclenick]

Yep. I've put increase when I meant decrease many a time. All it takes is fingers on autopilot.