Author Topic: .223 Twist Rate?  (Read 36896 times)

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bulldog136

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.223 Twist Rate?
« on: July 22, 2009, 09:57:42 AM »
I was reading my Lyman manual a couple of days ago and now I'm not sure of the bullet weight compared to the barrel twist on an AR series rifle as far as a good match up. Do you have any good reference material on this subject?
« Last Edit: February 11, 2010, 11:49:23 AM by Frisco Pete »
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Frisco Pete

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.223 Twist Rate?
« Reply #1 on: July 22, 2009, 01:21:53 PM »
You can find guides to what bullet/what twist elsewhere, but I will give you my 2 cents worth here and hope it makes sense. 

One thing to visualize in the following discussion is that if you take a certain weight ball of Play-Doh and another heavier ball and roll them out to the diameter of a pencil, then the heavier ball of Play-Doh will be the longer one as well.  If you shape a boattail and a pointy nose, the extra weight will have to go somewhere and so it will get longer again.

What the various manuals are referring to in .223 is the twist rate necessary to stabilize bullets in order to avoid them hitting the target sideways, if at all ("keyholing") as this picture by Molon on AR15.com illustrates:

This is what happens when you fire the 75 grain A-MAX from a 16" barrel with a 1:9" twist.


The reason has everything to do with the length of the bullet.  The longer the bullet, the faster the twist rate is required to stabilize the bullet in a nice stable spin (like a top).  While this is generally related to bullet weight, once again there are exceptions if a bullet is very short or long for the weight.  The main reason we go by bullet weight is because that quantity is known, and obviously a heavier bullet in a given caliber is longer.  Boattail bullets are longer than flat-base bullets as well.
Originally the .223 was developed from the .222 Remington which uses a 1-14" twist to stabilize 50 and 55 grain bullets.  So the very first Armalite AR15s had a 1-14" twist rate.
The long .223 55-gr tracer round was found to not stabilize properly in cold (lower velocity) arctic conditions with that twist so the .223/5.56 twist was changed to 1-12" and remained that way throughout the Vietnam war and into the early 1980s.  This is also the standard twist for bolt-action sporters until recently.

When the European NATO allies decided to adopt the 5.56mm in the late '70s - early '80s, they decided they wanted a heavier bullet that would penetrate Soviet body armor at extended ranges better so they developed the SS-109 62-gr FMJ-BT bullet.  This long bullet required a faster twist (at least 1-10").  When the U.S. adopted their SS-109 version, called the M855, they wanted sure stabilization and went with a fast 1-7" twist in the new M16 A2 rifle.  While perhaps it is overkill (or overstabilization) for the lighter bullets, they will hit the target point-on with mil-spec accuracy.  Sport shooter detractors felt this twist rate degraded accuracy of the lighter bullets so a compromise twist rate of 1-9" became popular in AR15s intended for civilian use.  You could get decent accuracy from a wide range of bullet weights, omitting only the heaviest match bullets.  On the other hand - National Match shooters used the fast-twist 1-7" to push new specialized ultra-long heavy BTHP match bullets of 75 to 80+ grains with their excellent ballistic coefficients to dominate 600 yard target shooting from the "mouse gun" and obsolete the .308 M14/M1A.

As alluded to above, the twist rate can have an effect on accuracy, with the ideal being a rate that stabilizes the bullet without adding any unnecessary rotational stress - once again there can be exceptions because of barrel smoothness and bullet quality so this is only a general guide, not gospel for every rifle: 
With 1-12" twist you are most like to find happiness in the 40 (plastic tipped) to 55 grain range. 
With 1-9", 50 to 69 grains with certain exceptions like 40-grain plastic-tipped bullets (more on this next). 
With 1-7" 55 boattails to 77 grains.

Another factor that plays into twist rate is bullet imperfection.  The shorter a bullet is for its weight, the easier it is to stabilize, even if the bullet's not perfectly balanced.  This is partly why early .22 centerfires had relatively gentle rifling twists, like 1-14" or 1-16" for the Hornet.  Even a slightly imbalanced bullet will often shoot fairly well if not spun too rapidly.  With the advent of modern bullets made on precision machinery, long range match shooting with the AR platform, and the change to the SS109/M855 bullet, twist rates have become much quicker for the .223 Rem.
The plastic-tip bullet revolution increased both ballistic coefficient and explosive expansion.  The also became available in lighter than traditional weights, such as 40-grain.  These bullets are now made on much more precision machinery as well so they are better balanced.  This meant that lighter bullets shot accurately even in rifling twists that, according to older theory, were way too fast.  It turned out that the big problem wan't rifling twist but the older bullets.  Today it is common for 40-gr plastic-tipped bullets to shoot extremely well even in 1-9" twists. 

To prove how well light plastic-tipped bullets like the V-Max or Ballistic Tip shoot in fast twists - a recent Handloader magazine test found that 40-gr plastic-tipped bullets (these are a bit longer BTW) shot the best of all lighter loads tested in a 1-8" Rock River Varmint AR15 and a 1-9" Savage bolt, with the accuracy edge going to the RR AR15.  These were 1/2 MOA groups.  However we need to remember that it was a smooth match-grade barrel with a long concentric high quality 40 grain bullet.  Shorter 40 and 45 grain SP bullets intended for the Hornet shot very poorly but were plenty stable!

The 1-12" twist stabilizes spitzer bullets from under 40 grains to 60 grain flat-bases.  An exception would be the short Speer 70-gr semi-spitzer that was made to stabilize in 1-12".  Designed for 55-gr M193 FMJ-BT bullets.

The 1-10" and 1-9" twist stabilizes spitzer bullets from 40 grains to 70 grains.  An exception is that most 1-9" rifles will stabilize the shorter 75-gr Hornady BTHP, but sticking with the popular 68-69 grain BTHP match bullets as the heaviest in a 1-9" for all conditions is a sure thing for stabilization and accuracy.

The 1-8" and 1-7" twist stabilizes spitzer bullets from 40 to 80 grains.  One issue with the longer versions of 75 and 80 grain bullets is that they are so long that they cannot be loaded to magazine length and must be single-loaded into the rifle.  The illustrated 75-gr Hornady A-Max is one of that type.  Standard military M855 is a 62-gr FMJ-BT and specialized military long range match is Mk262 77-gr BTHP match (Nosler or Sierra).

Like much of what I have found in my years of reloading and shooting - there are no absolutes (well, we are quite sure that 75 A-Maxs won't shoot in a 1-9"!) but sticking in the general guidelines should give you an idea of a bullet weight to try.  Other considerations are intended usage, length of bullet factoring in that boattails or plastic-tipped bullets are longer than standard flat-bases, and really long noses like the VLD or A-Max are really long bullets.

Another variable is the ALTITUDE that you are shooting at.  Atmospheric pressure has a definite effect, primarily seen at different altitudes:  Less twist is required to stabilize a bullet in the thin air of higher altitudes than at sea level.  So stuff I can get away with at 6000 feet may not work at all at sea level.

In my shooting I have found that 55s shot better than 60 in my 1-12" bolt and that the plastic-tip Nosler Ballistic Tip BT 40-grain shot best of all against my own judgement.
In my 1-9" ARs both 55-gr Hornady SP (flat-base) and 55-gr Hornady FMJ-BT bullets shoot well.  60 V-Max not as well (could be powder used) and 68 and 69 grain BTHP match bullets shot MOA.  I have never tried the 40s in these rifles.
I don't own a 1-7", but would if National Match-type shooting or use of Mk262 defense loads were intended.   
« Last Edit: March 08, 2012, 03:03:22 AM by Frisco Pete »
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bulldog136

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.223 Twist Rate?
« Reply #2 on: July 22, 2009, 01:30:42 PM »
Holly $#!t, can I just hook up a terrabyte external harddrive to your brain and download everything? I spent two days browsing thru books and walked away with a headache and lots of confusion. You summed up exactly what I was looking for in that last post. Oh, and it made perfect sense. I knew somethings, i.e. keyholing but  you answered the longer the bullet the faster the twist rate question for me.

This, for me at least, has been the best post on here so far.
Thanks a million.
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Danjal

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Re: .223 Twist Rate?
« Reply #3 on: October 15, 2009, 07:13:44 PM »
Nice write up frisco..and thats some serious keyhole issues.
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Hodgie

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Re: .223 Twist Rate?
« Reply #4 on: October 15, 2009, 07:55:23 PM »
Great information, made it so even I can understand it.

saintdeath

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Re: .223 Twist Rate?
« Reply #5 on: November 03, 2009, 02:26:25 PM »
Also note on some Colt AR's with a 1:12 twist 40 gr. Hornady Are EXCEPTIONAL to use as well but a Colt with a 1:10 (although there are FEWER!) Prefers a 40gr. to 50 gr Better ! As Guns and Ammo Book of the AR 15 stated:

AR's with a faster twist (i.e. 1:7) prefer and stabilise a heavier bullet up to 80 plus grains.
AR's with a slower twist (i.e. 1:12) will stabilise a lighter Bullet from 40gr. to the 55 gr. range.
Some AR's are now being developed to carry a 1:6.5 rifle twist and even a 1:6 for heavier loads (bullets 85 grains and over).

Some cases have been observed where bullets of a lighter weight (i.e. 55 grains) have been shot in a 1:7 and vaporized before hitting the target. But also some rounds of the same weight have performed well in a 1:8 twist barreled rifle!
And in other cases rounds with a 72 grain bullet could not stabilise almost at all in a 1:12 barrel and made the bullets tumble right out of the barrel after firing! .....:---David Fortier G&A Book of the AR 2007

FRISCO PETE WAS SPOT ON IN HIS STATEMENT!!!! ANOTHER REASON THIS PLACE IS AWESOME ! KEEP UP THE GREAT WORK ! ! ! ! !
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shmeddie

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Re: .223 Twist Rate?
« Reply #6 on: November 10, 2009, 07:06:42 PM »
wow, friscopete you answered all tthe questions I was going to ask. haha

saintdeath

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Re: .223 Twist Rate?
« Reply #7 on: December 21, 2009, 10:23:03 PM »
Barrel Twist Rate:          1:14"         1:12"            1:9"                1:8"            1:7" or 1:6.5"
Max Bullet Weight :     55gr FB       65gr FB        73gr BT           80gr BT          90gr BT VLD


HOPE THIS HELPS TOO!
Trespassers Will Be Shot !
Survivors Will Be Shot Again !
Liberals Should Be Annihilated !
Veterans Must Be Respected !
Protect the Constitution !
Teach The Youth GUN SAFETY!
Keep America ARMED!

nhblaze

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Re: .223 Twist Rate?
« Reply #8 on: December 02, 2010, 03:56:37 PM »
This is my guide for twist rate for any caliber

It's a short version of the Greenhill Formula

OLD but it will get you close.

Formula for bullet velocities UNDER 2800 fps

150 X bullet diameter squared divided by bullet lenght

Formula for bullet velocitis OVER 2800 fps

180 X bullet diameter squared divided by bullet lenght

So lets try a .243 Winchester for example, at under 2800 fps

.243x.243= .059  bullet diameter squared

150 x .059 =  8.850 divided by the lenght of the bullet I picked a .750" long bullet

8.850 divided by .750 = 11.8  or a 1 in 12 twist


It's not a perfect science but will work.

Also try this calculator http://www.bergerbullets.com/litz/TwistRuleAlt.php

« Last Edit: December 14, 2012, 02:44:07 PM by nhblaze »
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saintdeath

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Re: .223 Twist Rate?
« Reply #9 on: December 04, 2010, 09:15:33 PM »
There ya go ! NOT DEAD ON BUT SHEEESH CLOSE ENUFF  8) ! ROUND IT OFF PEOPLE !  :roll: :lol:
Trespassers Will Be Shot !
Survivors Will Be Shot Again !
Liberals Should Be Annihilated !
Veterans Must Be Respected !
Protect the Constitution !
Teach The Youth GUN SAFETY!
Keep America ARMED!

blueridgenc

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Re: .223 Twist Rate?
« Reply #10 on: April 17, 2011, 10:04:35 AM »
Great article on the bullet weights and twist rate of the ar barrels. I am now armed with the Knowledge of experience and will save time and money..

Clark

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Re: .223 Twist Rate?
« Reply #11 on: November 14, 2011, 07:22:10 PM »
I have built some 223 rifles with 22LR barrels with 1 in 16" twist.

They stabilize 52 gr bullets but not 55 gr.

I mostly shoot 35 gr Vmax with a wimpy 15 gr Blue Dot of 3500 fps, to kill ground squirrels.

wolfgang2000

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Re: .223 Twist Rate?
« Reply #12 on: September 06, 2012, 03:26:51 PM »
Frisco Pete great right up.

A question.  I've read that the 1n7 twist was needed to stabilize the longer tracer round.  Correct?

Also just a note of intrest. The original twist in the M16 (the test models) was 1n10.  This allowed the bullet (55 grn) a certain amount of yaw in flight, thus it was easily upset upon impact.  However at that twist the bullet was unstable in Arctic air, thus it was changed to 1n12.
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Frisco Pete

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Re: .223 Twist Rate?
« Reply #13 on: September 07, 2012, 08:27:08 AM »
I'm not positive about the reason for 1-7" for the 62-gr as 1-9" is plenty, but it probably has something to do with the longer trajectory-matched tracer - but for sure that is the reason that the original M16 was changed from 1-14" to 1-12" twist.
You probably meant 1-14" instead of 1-10". 1-14" is the twist rate of the parent .222 Rem and sibling .222 Rem Mag, and other .224" dia rounds of the era except for the Hornet.

One early SEAL in 'Nam, Doc Watson "Pointman", reported that IHO from experience with both, the 1-14" twist rate did more damage on contact as the bullet was less stable - or so went the explanation. However other sources refute this idea. So the effect of twist or over-stabilization on terminal effectiveness is debatable.
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wolfgang2000

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Re: .223 Twist Rate?
« Reply #14 on: September 19, 2012, 02:16:33 PM »
Thanks for the correction Frico.  You are correct 1n14 is what I ment.
“The key is to hit them hard, hit them fast, and hit them repeatedly. The one shot stop is a unit of measurement not a tactical philosophy.”  Evan Marshall