AR Barrel twist: Bullet stability or accuracy

[no_problem]

When the AR began life back in the 60's, they came with 1:14" twist. This twist rate, when used with a 55 grain bullet, barely stabilized the bullet. The result was that the bullet tumbled upon impact, causing great bodily harm. As the AR modernized, barrels got shorter, and twist rates got faster. Today, 1:9" and 1:7" are common. These new faster twist rates stabilize the bullets much more, in some cases lighter bullets in these twist rates might twist so fast that they heat up and begin to deform from the heat of the fast twist.

There is little argument that today's faster twist rates do much more to stabilize the bullets, increasing their accuracy at farther distances. But the faster twist rate seem to negate the heavier damage that a tumbling slower twisting, lighter bullet can do.

 

[no_problem]

Edited to add:

The original AR's had 1:14" twist rates, but they soon changed to the 1:12" twist rate which saw service throughout the Viet Nam conflict. The 1:12 twist rate seems to stay in the M16 A1.

 

[JWarren]

You failed to mention that the faster twist barrels were developed to stabilize heavier grain bullets.

It becomes an apples and oranges discussion.


-- John

 

[wilson]

Dont all FMJ bullets tumble.

 

[JWarren]

Dont all FMJ bullets tumble.

Not necessarily.



-- John

 

[Soybomb]

These new faster twist rates stabilize the bullets much more, in some cases lighter bullets in these twist rates might twist so fast that they heat up and begin to deform from the heat of the fast twist.

Lighter bullets may spin so fast they fly apart from the force exerted on them, it is not a heat related failure.

But the faster twist rate seem to negate the heavier damage that a tumbling slower twisting, lighter bullet can do.

Twist rate has nothing to do with the bullet tumbling once it enters a body. Twist rate is not a component of wounding.

 

[Coltdriver]

Go to http://www.ammo-oracle.com/body.htm to get the scoop on AR ammo.

 

[no_problem]

Good observation, JWarren. Newer, faster barrel twists ARE designed for heavier bullets. So today's AR, with it's heavier bullets flying at a faster twist rate, makes for a heavier and more accurate projectile that has a longer distance. But is that a more effective battle rifle round than a slower, lighter bullet that may tumble once it hits it's target?

Also, today's heavier bullets which flies faster, more stable and farther, and also has no cannelure, is less likely to break up upon impact. The older 55 grain bullets had a cannelure, and sometimes tumbling bullets may split or break up at the cannelure thus creating two projectiles.

 

[Soybomb]

Good observation, JWarren. Newer, faster barrel twists ARE designed for heavier bullets. So today's AR, with it's heavier bullets flying at a faster twist rate, makes for a heavier and more accurate projectile that has a longer distance. But is that a more effective battle rifle round than a slower, lighter bullet that may tumble once it hits it's target?

Also, today's heavier bullets which flies faster, more stable and farther, and also has no cannelure, is less likely to break up upon impact. The older 55 grain bullets had a cannelure, and sometimes tumbling bullets may split or break up at the cannelure thus creating two projectiles.

The heavy OTM bullets that are popular for defensive use today upset and fragment easier than the usual mil rounds making them a better choice for short barreled weapons.

There is a lot of missinformation you seem to have. I would strongly recommend you register at http://www.tacticalforums.com/cgi-bin/tacticalubb/ultimatebb.cgi and read the terminal effects forum. There is a lot of really great work there by people like Dr. Gary Roberts, and I mean quality work too, stuff's that been published in peer reviewed journals following proper procedures.

 

[no_problem]

"There is a lot of missinformation you seem to have. "

Thank you for your opinion and your sources. I too have my sources. These comments resulted from the article in Tactical Shooter magazine, Vol 3, No 7.

I am not sure what I did to deserve an insult, but I feel that I have brought a legitimate and pertinent discussion to bear.

 

[Coronach]

Thank you for your opinion, but I have my sources. These comments resulted from the article in Tactical Shooter magazine, Vol 3, No 7.

Heh. OK. Care to quote?

Mike

 

[Bartholomew Roberts]

I am not sure what I did to deserve an insult

Failing to agree with your unnamed source of information is not an insult.

 

[no_problem]

To avoid plagirizing, please purchase the magazine yourself. Tactical Shooter, Vol 3 No 7

The discussion in the copyrighted article begins on page 79.

The topic is about the Armalite/Colt design that barely stabilizes bullets of typical weight and length and discussions of complete disembowelment with torso hits and heads and arms being blown completely off

 

[no_problem]

<< Today, 11:32 PM #12
Bartholomew Roberts
Moderator


Join Date: 12-26-02
Location: Texas
Posts: 9,327

Quote:
I am not sure what I did to deserve an insult
Failing to agree with your unnamed source of information is not an insult.>>

Point taken, No Problem.

Sources and citations from copyrighted materials provided.

Tactical Shooter magazine
222 McKee Street
Manchester Ct. 06040
Phone (860) 645-8776
President: Dr. Richard Maretzo

 

[Coronach]

What is the date of this publication?

Mike

 

[no_problem]

Aug 2000

 

[ny32182]

Plagarism means that you try to take credit for the writings of others. If you quote it in context and don't try to take credit for writing it, it isn't plagarism.

When the AR began life back in the 60's, they came with 1:14" twist. This twist rate, when used with a 55 grain bullet, barely stabilized the bullet. The result was that the bullet tumbled upon impact, causing great bodily harm. As the AR modernized, barrels got shorter, and twist rates got faster1:9" and 1:7" are common. These new faster twist rates stabilize the bullets much more, in some cases lighter bullets in these twist rates might twist so fast that they heat up and begin to deform from the heat of the fast twist.

There is little argument that today's faster twist rates do much more to stabilize the bullets, increasing their accuracy at farther distances. But the faster twist rate seem to negate the heavier damage that a tumbling slower twisting, lighter bullet can do.

After more extensive reasearch, you will find that it is widely accepted that the weight distribution of a bullet determines if it will tumble, and its construction determines if it will break apart during a tumble. Twist rate alone has nothing to do with it.

Also, modern heavier bullets tend to be fired a relatively lower velocities than the M193 type 55gr rounds. Their construction allows them to break up at lower velocity than M193, which is why they are said to work well in short barrels...

 

[mc223]

Link to more info:

http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html

And for tumbling:

http://anarchangel.blogspot.com/2007/02/terminal-tumbling.html

 

[Hunter0924]

As I understand twist rate it is not really the bullet weight but length that determined ideal twist.
Longer bullets (usually heavier but not always) require a faster twist to stabilize.

 

[Soybomb]

Thank you for your opinion and your sources. I too have my sources. These comments resulted from the article in Tactical Shooter magazine, Vol 3, No 7.

I am not sure what I did to deserve an insult, but I feel that I have brought a legitimate and pertinent discussion to bear.

There is no insult intended I'm just pointing out that articles published in wound ballstics jounals by the experts in the field today contradict what you read in an 8 year old magazine article. I then pointed you to a source where you could find up to date research based on good scientific methodology. What you do with that information is entirely up to you. More importantly to me is that the 3rd party see there is another side to the story and can make an informed choice of their own.

At the very least I would urge you to examine some of the other claims maid by the article such as the one that says heat is what causes rounds to fly apart. If that is wrong, do you trust the other stuff in the article?

 

[whistler]

how could anyone design a bullet to tumble on impact when the angle of impact will always be unkown?
rate of twist depends on length and speed of a bullet.

 

[Soybomb]

how could anyone design a bullet to tumble on impact when the angle of impact will always be unkown?

This is addressed well in the link above, its the natural behavior of the bullet because its center of gravity is toward the rear. The spin from the rifling can keep the bullet stable in flight, but not in tissue. When it hits tissue the bullet wants to yaw so the heavier end of the bullet can be in the front. For the rounds we're likely talking about the velocity will be great, the forces on the bullet stronger than it can take, and then the bullet fragments. The back wants to swap ends with the front and the angle you fired it into the tissue at doesn't change that.

 

[rangerruck]

once again, we can thank the air force, seems they screwed us all 100 yrs ago, When we were first thinking of the 6mm lee-navy. then , when everyone first liked the ar, before it got all screwed up, the air force decided that the ar didn't work past 10 degrees below, and would not adopt it, until changes were made. This brought the first twist change down to 1/12, which as you know , really stabilized the 55 grain bullet, which pissed off the testers, the origional designers, the first dudes with feet on the ground in Viet Nam, etc.,...
If you want to read a great article, get the latest issue of Small Arms Revue. There is 1 of a 3 part series of intervues with Jim Sullivan; He was one of the dudes who worked on the Stopette,ar10, stoner 62/63, chiclette gun, chain gun, ar 18, ar 16, etc. He is still griping about problems, that congress has never bothered to make sure got fixed, on the ar's.

At least we got the 220 Swift out of the 6mm Lee.

 

[Bartholomew Roberts]

But the faster twist rate seem to negate the heavier damage that a tumbling slower twisting, lighter bullet can do.

ny32182 is correct. This is mythology. In order to stabilize a bullet in a mostly liquid medium (and the human body is mostly water), you would need rifling that looks like a machine screw. The difference between 1:14 and 1:7 is pretty insignificant.

The good thing is you don't have to take my word for it either. Just go find a round that fragments in gel or water filled jugs in 1:14 and then fire it through a 1:7 - it will fragment there too.