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Twist rate of 223 and bullet accuracy

This is a discussion on Twist rate of 223 and bullet accuracy within the Ruger Bolt Action forums, part of the Rifle & Shotgun Forum category; Just read an article in the april issue of SHOOTINGXXX'S gun porn. It was very interesting to see what bullets require what twist to shoot ...


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Old February 19th, 2012, 01:55 AM   #1
 
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Twist rate of 223 and bullet accuracy

Just read an article in the april issue of SHOOTINGXXX'S gun porn. It was very interesting to see what bullets require what twist to shoot accurately. Further, it appears the 1 in 9 twist works rather well for most bullet weights in 223 Remington. Cast bullets were not mentioned, but they are not a good bullet for semi-auto. However, for a single shot or bolt action, it would have been interesting to see how they react to different twist. For those interested in such stuff as accuracy, it was a good read.

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Old February 22nd, 2012, 02:52 AM   #2
 
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Yes, a 1 in 9twist will handle up to 75 grain bullets. My 223 Ackley Improved is a 1 in 8 twist and shoots the 80 grain Berger VLDs and Sierra SMKs very well.

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Old February 22nd, 2012, 06:57 AM   #3
 
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Something to remember with the 9-twist is bullet destruction.
Savages bolt guns are ALL 9-twist, cheap bullets don't like that.
If you like to shoot full power loads(no, not over-loaded) then you can't use:
SXPX, or Dogtowns. They have a RPM ceiling of around 240,000rpm, barrel smoothness dependant.

Midway STILL touts the 4,000 fps limit, which really is of no help. Velocities are mostly irrelevant to a bullet, it is the spin that destroys them.

MV * 720 /twist = RPM

So as an example only: 50gr dogtown ONLY going 3000fps * 720 /9 = 240,000rpm. Depending upon your particular barrel, that maybe to fast, it may not quite be.
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Old February 22nd, 2012, 10:41 AM   #4
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Yep. Another example that one might have trouble with is the 50-grain TNT. Going full-speed, they may or may not want to hold together. The 55-grain High Velocity TNTs do fine, though.

On the 75-grain thing: You guys might want to actually shoot the 75 AMAX at long range out of the 9-twist bbls. Depending upon a lot of things, you might find that they're not actually quite stable from a 9. It'll probably take shooting at long range to really see that, though. As with the varmint bullets, some rifles might make 'em work well enough, but others won't.
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Old March 28th, 2012, 07:31 AM   #5
 
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I read that a 1-8 barrel has a maximum velocity of 3200 (apprx) fps because of the spin rate and any faster it will start to destroy the jackets on bullets. Is this true?
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Old March 28th, 2012, 12:52 PM   #6
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From what I understand, and I'm no expert, heavier bullets like a faster twist rate for longer distances. I think most ARs are 1-9 while some are 1-8 and 1-7, for heavier bullets, 68 grain and up.
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Old March 28th, 2012, 01:00 PM   #7
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There is a lot of misconceptions about twist rates ... even many of the gun rags don't get it right.

First and foremost ... twist rate is not the determining factor for accuracy ... it merely has to do with downrange bullet stability. Yes, there is a connection but not what you might think.

Let's start with the basics .... bullets need to be spun at the proper RPM (see RPM formula in Darkker's post) so they will maintain stability down range. As noted in the formula, two things affect bullet spin rate ... the barrel's twist rate and the velocity of the bullet. As everyone knows ... velocity drops down range as air friction impinges on the bullet. What most people don't realize is .... the bullet's spin rate also deteriorates at about the same rate as velocity. At some point down range, bullet spin rate (RPM) will slow down enough where the bullet will begin to wobble and soon after will begin to tumble. Until the bullet reaches a distance down range where it begins to wobble, accuracy is not affected, but of course after the bullet becomes unstable, accuracy will go south.

Bullets: With the exception of hollow base wad cutters or dual ended wad cutters (which are not used in anything but handgun loads) all bullets are base heavy. Think of this ... when you throw a dart tail first, what happens? The dart's nose is much heavier than the tail so soon after you throw it, the nose and tail swap ends and the dart continues nose first. The same basic thing happens with bullets ... the heavier base tries to swap ends with the nose from the time it clears the muzzle. The way to keep the bullet going nose first is to spin it and create a "gyro effect", much like a toy top. As long as the toy top is spinning fast enough, it will remain upright but as soon as it slows down, it will start to wobble and finally topple. Bullet do the same thing.

Besides velocity and twist rate, there are three more things that affect bullet stability, the first being ballistic coefficient (BC). The higher the BC, the farther the bullet will travel before air friction slows the RPM to a point of instability. The second issue is bullet length ... because longer bullets are usually heavier, we generally use bullet weight to determine the proper spin rate. With hollow points, a lighter bullet could actually be longer than a heavier bullet so using bullet weight is not always the best gauge for twist rate. The third and probably the most important issue is bullet "quality". Match grade bullets are way more uniform than hunting bullets ... bulk bullets are the worst. If you buy a box of bulk bullets and weight them, very few actually weigh the advertised weight ... most will be light. When bullets are manufactured, there is often a bubble or void in the lead core, which will make the bullet fly like a whiffle ball. Match grade bullets have very uniform weights so they rarely have voids in the core and fly true. Out of balance bullets not only lose stability at closer distances, they are never as accurate as higher quality bullets at any distance.

A little history for the 223 Rem .... When the 223 Remington cartridge was first introduced, it was chambered in a bolt action 24" barrel rifle with a 1:12 twist rate. At the time, a 55 gr FMJ bullet loaded to 3260 fps was the standard. This twist rate and velocity would keep a 55 FMJ stable to at least 300 yards ... farther with better grade bullets. When the M-16 was first introduced, it also fired a 55 gr FMJ 5.56, but the barrel was 20" and the twist rate was 1:14. Turns out, a 1:14 twist rate coupled with slightly lower muzzle velocity from a shorter barrel would not keep bullets stabilized past 200 yards. Long range accuracy was very poor so it was changed to 1:12 twist rate. This extended the range to at least 300 yards; however, barrel life was reduced by about 50%. Firearms engineers found lining the bore with chrome extended barrel life with the faster twist rate better than the old 1:14 barrels. During the Viet Nam era, 55 gr bullets weren't heavy enough to get the job done so the military went to 62 gr bullets. Of course the 1:12 twist rate wasn't fast enough to keep the heavier bullets stabilized at longer distances so the M-16 twist rate was changed to 1:10.

For no good reason, the civilian market tends to follow the military so faster twist rates have become popular in all 223 Rem rifles. What most people don't know is faster twist rates are usually counter productive .... accuracy is not as good and barrel life is way shorter. Besides, only long range target shooters really need a fast twist rate ... the rest of us seldom shoot at distances much farther than 300 yards.

Stabilization: Heavier bullets can't be driven as fast as lighter bullets without exceeding max chamber pressures. Likewise, shorter barrels won't develop as much velocity as a 24" barrel so the "fix" for both is to increase the twist rate. If a bullet is not spun fast enough (under stabilized), it will still be very accurate out to several hundred yards but a longer distances, it will become unstable and lose accuracy. If a bullet is spun too fast (over stabilization) it will cause the bullet to develop a spiral cork screw like bullet path. An over stabilized bullet at just 100 yards will open groups up to 4" or more, compared to a properly stabilized bullet. Besides accuracy issues, the barrel won't last very long if you shoot lighter bullets at higher velocities in a fast twist rate barrel. There is no one twist rate that works well for all bullet weights so the best plan is to figure out what bullet weight you want to shoot then buy a barrel or gun to match.

For a 45~55 gr bullet, a twist rate of 1:12 is optimum. This will get you the best accuracy and stability out to 300~400 yards or more. For 60-62 gr bullets, you need a 1:10 twist rate, which will also be good out to the same distance. For 60~62 gr bullets at extended distances out to 600~750 yards, a 1:9 twist works best. For heavy bullets in the 70~75 grain range, a twist rate of 1:8 will stay stable as far as 1000 yards.

As I mentioned before, there is absolutely no advantage using a faster twist rate than needed. All it does is wear the barrel faster and lose accuracy.
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Old March 28th, 2012, 07:17 PM   #8
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Rifle bullets NEVER become unstable during flight due to rpm reduction. NEVER. In fact, their gyroscopic stability factor increases as they travel downrange. This is because the drag against the bullet's rotation is dramatically lower than the drag against its flight downrange. Accordingly, Rifle bullets DO NOT tumble because their gyroscopic stability factor decreases downrange. If a rifle bullet tumbles, it is because of very specific aerodynamic happenings as the bullet approaches and passes through the trans-sonic speed range (barring excessive mass eccentricity, jacket separation, or other physical defects).

See this post over at Shooter's Forum for a good, if slightly tangential and very long, post related to this.

Last edited by MZ5; March 28th, 2012 at 07:19 PM.
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Old March 28th, 2012, 08:50 PM   #9
 
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....my AR ...223... Wizard has said... * A good rule of thumb is that 1:9 will stabilize bullets in the 45 to 62 grain range, and 1:7 will stabilize bullets in the 55 to 77 grain range.

I have a 1:7........so far...so good.....
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Old March 28th, 2012, 11:29 PM   #10
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MZ5, I respectfully disagree with your statement:
Quote:
Rifle bullets NEVER become unstable during flight due to rpm reduction. NEVER. In fact, their gyroscopic stability factor increases as they travel downrange.
I would agree with your statement if a bullet was fired in free space, however here on earth, we have this strange thing called "air" that applies considerable friction on any moving object. Air friction does three things to a bullet ... it slows down velocity, it slows down the RPMs, and it causes the bullet to heat up. Let's dwell on the spin rate for now ... just like a gyroscope or toy top, friction on the bearing surface causes the spin rate to slow down and at some point, the spin rate will no longer support the gyro effect and will cause the toy top or gyroscope to lose stability. At first, the gyro starts to yaw a bit, then it starts to wobble, and finally it falls over. Bullets do exactly the same thing.

Here's some test data I remember from when I worked at the DOJ lab. The test rifles (several of them) were installed in test jigs, much like a Ransom Rest for handguns. This totally eliminated the "human element". All guns were chronographed at 10 meters and average velocities were virtually identical. We started with an M-16 with a 1:14 twist rate barrel, 55 gr bullets 5.56 mm, Ball, M193 ammo. At 100 meters, the bullets made perfectly round holes in the paper target and were exceptionally accurate (sub-inch groups). This indicated the bullets were very stable. At 150 meters, there were very slight oval shaped holes in the target. Groups were still quite good ... about an inch. This indicated bullets were still quite stable. At 175 meters, oval holes were more pronounced and group size increased to about 3". This indicated bullets were now beginning to wobble. At 200 meters, many of the bullets were creating "key holes" in the target. Groups spread to more than 6". This indicated bullets were tumbling. Finally, at 250 meters, groups spread to more than a foot and all bullet holes were keyholes or wide ovals ... indicating total instability.

The same tests were conducted using M-16s with a 1:12 twist rate and the same M193 ammo. Out to 250 meters, all bullet holes were nice and round. Groups were proportional to distance and averaged 1/2 MOA. This indicated bullets were maintaining perfect stability. At 300 meters, bullet holes started to oval and groups spread to about 1 MOA (just over 3"). At 350 meters, there were key holes in the targets and groups spread to 5 MOA (about 16").

During the tests, a 10 shot string was chronographed at each distance. Both twist rates averaged the same velocity up to 150 meters. At 175 meters and beyond, the 1:14 twist rate rifles chronographed slower than the 1:12 twist rate rifles.

Same ammo, same barrel length, same muzzle velocity, same basic guns ... except for the barrel's twist rate. Winds were dead calm for all tests. So what did this prove? It proves air friction slows the bullet's spin rate and when the spin rate drops below stability threshold, bullets will become unstable, start wobbling, and finally begin to tumble. It also indicated a loss of velocity with the slower 1:14 twist rate rifles at distances beyond 175 meters. That's because unstable bullets aren't as aerodynamic as stable bullets.

The bullet's ballistic coefficient dictates how well a bullet can overcome air friction, both in velocity loss and in spin rate. M193 bullets are FMJs with a boat-tail and have a BC of .255, which is pretty grim when compared to a 7mm or 30 cal where BCs can be .600 or more. Bullet spin rates decays nearly proportional to velocity loss. I am not aware of any device like a chronograph that will measure spin rate down range but it is easy to compute. I don't remember the actual chronograph data from the above tests so I plotted the M193s using Ballistic Explorer. Here's the results: Muzzle velocity is 3240 fps At 100 meters it drops to 2820 fps, 2624 fps @ 150 meters, 2530 fps @ 175 meters, 2437 @ 200 meters, 2257 fps @ 250 meters and 2085 fps @ 300 meters.

Using the formula for RPM, (muzzle velocity in fps times 12 inches divided by twist rate in inches, times 60 seconds in a minute), we find the spin rate for a 1:14 twist is 3240 times 12/14 times 60=166,628 RPM at the muzzle. The spin rate for a 1:12 twist is 3240 times 12/12 times 60 = 194,400 RPM at the muzzle. With the 1:12 barrel, the above tests show 300 meters is the threshold for instability. If we divide the velocity at 300 yards by the velocity at the muzzle we will get the percent of retained velocity (2085 divided by 3240 = about 64%). So the spin rate has also slowed down to about 64% of what it was at the muzzle. For the 1:12 barrel, .64 times 194,400 RPM at the muzzle = about 124,416 RPM. We can then conclude it takes about 125k RPM to maintain bullet stability. For the 1:14 twist rate, we can then compute the max range where the bullet loses stability (less than 125k RPM). At 175 meters, velocity has slowed down to 2437 fps (about 75%) so RPM would be .75 times 166,628 RPM at the muzzle or 124,971 RPM, which is amazingly close to 125k RPM. As you can see, the same exact bullet in a 1:14 twist barrel loses stability at 175 meters, whereas a 1:12 twist barrel will lose stability at 300 meters ... all due to air friction slowing down the bullet's spin rate.

I'm in total agreement with the reference you posted. No where in the post did it refer to bullet spin rate decay, rather the author was trying to explain the phenomenon of spiral effect (AKA corkscrew effect). In a nutshell, a bullet spins in two dimensions ... one is on the bullet's axis and the other is a spiral that starts as soon as the bullet leaves the muzzle and continues to grow in diameter to several inches at about 25 yards. The spiral then starts to dissipate, getting smaller and smaller as the bullet travels down range. At some point, the spiral will totally dissipate, leaving the bullet just spinning on its own axis. Long range shooters call this point "going to sleep". The longer the bullet, the more distance is required for the bullet to go to sleep. It is very common to see rifles with long bullets (a 6.5mm Swede comes to mind) shoot 3 MOA groups at 100 meters and 1 MOA groups at 200 meters or more. This defies common sense but it really happens due to the spiral effect. Normal 55 gr .224" bullets are quite short so they go to sleep in 50~75 meters. Longer 75 grain .224" bullets may take double that to go to sleep.
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Old March 29th, 2012, 02:29 AM   #11
 
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Greetings,

Very interesting information, and nice to see we have some experts with first hand experience. Thanks for all the comments...

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Old March 29th, 2012, 04:53 AM   #12
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Iowegan, please read Litz, Vaughn, McCoy, or any number of others for as much practical and mathematical (solid advanced calculus from McCoy) information on the topic as you like or can stand. You can open up to the whole thread on that page if you didn't see anything you liked in that post itself; there's additional discussion I didn't think people here would choose to wade through. However, the post I linked _does_ actually get at what you observed, but not directly so one must understand and think on ahead along a line of thought or two suggested in that post.

Your experiences/observations match what is predicted, with perhaps another factor or two thrown in, you just jump to the wrong conclusion as to the cause. McCoy is a great reference if math will help you understand. Litz will probably help most if practical analogy is best for you. Vaughn provides a mixture of both.

Your gyroscope analogy is significantly incomplete, though ironically if you take a moment to think it through it serves to support my point, not yours.

Last edited by MZ5; March 29th, 2012 at 05:11 AM.
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Old March 29th, 2012, 07:44 AM   #13
 
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Quote:
Originally Posted by oneyeopn View Post
I read that a 1-8 barrel has a maximum velocity of 3200 (apprx) fps because of the spin rate and any faster it will start to destroy the jackets on bullets. Is this true?
NO. YES, MAYBE. There isn't a sweeping answer exactly, and barrel smoothness plays a big part of this to.

Hornady's manual(now 2? editions old) that first showcased the 223 WSSM achieved @ 4500 fps. This was done with a 8, or 9 twist.

Destruction due to centrifugal force, depends upon the RPM limit of a specific bullet.

When I specifically called and spoke with the bullet manufacturers, I was asking about RPM limits for 55gr .224" bullets.
Here is what I was told. Also this is posted on Shooters Forum.

Dogtown / Varmint nightmare - 240,000-260,000 RPM
Hornady VMax & SP(same rating) - 290,000 RPM
Sierra Varminters - 216,000 RPM
Sierra BlitzKings - 352,000 RPM
Speer TNT's(low velocity version) - 240,000 RPM
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Old March 29th, 2012, 09:56 AM   #14
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MZ5, I'm not an engineer nor do I pretend to be one. Back in the mid-60s, I was a lab tech in the DOJ firearms lab. As such, I worked with a lot of engineers that always seemed to have a "theoretical answer" to most any question. I always had the highest respect for them and usually they were right .... but sometimes common sense trumps theory. A good case in point is the George Greenhill formula for computing twist rate. Sometimes it works, sometimes it doesn't. The best solution always comes from actual testing. When the Greenhill formula was applied to the M-16 with M193 ammo, the optimum twist rate was computed to be 1:14 when in actuality, a 1:12 twist rate worked much better. Go figure!

In the M-16 tests above, there were several rifles used in each twist rate and as I stated above, same ammo, same muzzle velocity, same platform, same distances, same everything except the twist rates. The tests were done in typical controlled laboratory conditions where all parameters were the same except one ... twist rate. You can theorize all you want and quote famous sources but when you actually see the results, the conclusions becomes obvious. No doubt, there are other factors and maybe other scientific explanations but the actual proof points to spin rate decay ... nothing more, nothing less.

One thing I left out in the previous post was followup M-16 testing with faster twist rate barrels. I was not involved with these tests but I did see the results. A 1:10 twist rate barrel was used in similar lab tests. I don't know the details but I do remember accuracy was not acceptable at 100 meters with M193 ammo, which was due to "over stabilization".

Bottom line ... theories are great but you have to get out to the range and burn up some ammo to see how things really work.
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Old March 29th, 2012, 11:04 AM   #15
 
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Ok there is a lot of good information here, I had problems in Algebra in College (didnt need it in the Corps) so I majored in Music. Anyhow I started reading this thread because I am getting ready to start loading my own .223's. My barrel is a progressive twist 1-8 5R 16" long (yes an AR). I have been accurately shooting prairie dogs regularly out to 250 yards with my longest a 262 yard one shot kill. At the range the rifle is shooting ragged one holes at 50 yards, cloverleafs at 100 and 1" groups at 200 yards. I am scoped with a 4-16x40, I did have an 8-32x42 but the FOV was so reduced it was hard to find my targets sometimes. I started out with this rifle to be a fun gun, then I figured out how accurate the barrel is so I started extending my range and exploring the possibilities. The rifling starts out with a 1-11 and ends up at a 1-5 twist, basic math would make me think thats how they came up with 1-8 spin but I am not sure thats correct. So from what I have read I would be better off loading 70 grain quality bullets, instead of the 55 gr XM193's that I have been shooting, enabling me to shoot further distances accurately or am I missing something. Thank you gentlemen for all the information that I have read so far. But if I am missing something please set me straight.

Oh and MZ5...I read that post and other than the artillery shell reference, I was lost, but I tried to follow, in fact I read it twice and understand some of the principals but Wow, it gets real complex real fast.

Last edited by oneyeopn; March 29th, 2012 at 11:12 AM.
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