Author Topic: AR .223/5.56 barrel twist question  (Read 19846 times)

blackwolfe

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AR .223/5.56 barrel twist question
« on: February 21, 2009, 12:27:53 pm »
Ok, so I managed to get an AR before the election and the EBR and PIF frenzy started.  Have another that was put away that was built during the ban and is lacking some evil features.  Don't know much about them and have been asking a few questions here and getting good answers.  Although barrels seem to be hard to get, I have also been gathering some parts and tools to change the barrel on one and to build another, but I have a few questions on twist rates and bullet weights.

What bullet weights work best in the following twist rates and barrel lenghts?

     16" barrel
           1:7
           1:8
           1:9

     20" barrel
           1:7
           1:8
           1:9
Thank again for your help
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AR .223/5.56 barrel twist question
« on: February 21, 2009, 12:27:53 pm »

tombogan03884

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Re: AR .223/5.56 barrel twist question
« Reply #1 on: February 21, 2009, 12:29:30 pm »
If I remember right ( feel free too correct me you guys) the lighter the bullet the tighter the twist.
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D-Man

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Re: AR .223/5.56 barrel twist question
« Reply #2 on: February 21, 2009, 12:31:44 pm »
Actually a tighter twist can stabilize a much heavier bullet.  Such as a 1:7 is great for an 80grain bullet.  1:9 can handle the mid 50 grain bullets up to the 70 grain bullets.  1:8 is for the 60 grain to upper 70 grain bullets.  Those numbers are just based on my experience.

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blackwolfe

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Re: AR .223/5.56 barrel twist question
« Reply #3 on: February 21, 2009, 12:50:19 pm »
So will a lighter bullet stabilize in a tighter twist barrel, such as a 45-55 grain bullet shooting well in a 1:8 twist barrel.   I have heard people use the term that a bullet can "over stabilize."  I always figured a bullet was either stable or not stable. 
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Re: AR .223/5.56 barrel twist question
« Reply #4 on: February 21, 2009, 01:02:40 pm »
I don't know that you can "over stabilize," but if you put too much spin on a light bullet it will destabilize or even fly apart.  That might be what they were refering to when they said "over stabilize."

There is more to stabilization than just twist.  It is a combination of twist, bullet weight, and bullet speed.  That is where handloading gets interesting  ::)
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billt

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Re: AR .223/5.56 barrel twist question
« Reply #5 on: February 21, 2009, 01:36:04 pm »
In the AR-15 the most common twist rate is 1 in 9". This will stabilize up to 69 grains quite well. I've read that some get fairly good accuracy with up to 75 grains, but that's pushing it. for 70 grains and above a 1 in 7" twist is preferable, and even necessary. You can try some 75 grain ammo in a 1 in 9" twist and see how it performs. The worst is they will keyhole, but you'll only be out a box of ammo. For very light bullets from 45 to 55 grains a 1 in 12" twist will work. Many .22-250 barrels are rifled with a 1in12", or a 1 in 10" twist. The longer the bullet the more twist is required to stabilize it. Weight is a second consideration to length.   Bill T.
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Overload

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Re: AR .223/5.56 barrel twist question
« Reply #6 on: February 21, 2009, 03:05:17 pm »
I just read an article on this.  It's a (slight) misconception to say that bullet weight determines proper twist for bullet stabilization.  It's actually bullet length.  However, since heavier bullets are generally longer in the same caliber, you can make that statement and be basically right.

In general
1:7 will stabilize 55-77gr bullets
1:9 will stabilize 45-62gr bullets
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Re: AR .223/5.56 barrel twist question
« Reply #7 on: February 21, 2009, 03:47:40 pm »
Getting closer.  you need to look at the driving band of the bullet(the portion of bullet that comes into
contact with the barrel).  Heavier bullets being longer naturally have a longer driving band. I shot 75gr bthp
in my .223 for years with absolutely superb results.  55gr fmj have very little driving band. There are
variations depending on manufacturer.  I shoot frangible now and have almost the same driving band in
a 42gr bullet that I had in the 75gr bullet.
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m25operator

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Re: AR .223/5.56 barrel twist question
« Reply #8 on: February 21, 2009, 05:44:13 pm »
The twist rate is all about bullet RPM in the end, that's where barrel length and velocity with the same twist rate vary. Here is a pretty good article on the subject along with the formula for figuring it out.

http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/

On another note, most jacketed rifle bullets do not have driving bands,  Lapua, Norma and Barnes are exceptions, as they definitely do have driving bands in some offerings.

I have a custom Krieger barrel, 26" 1/6.5" twist for using 80-90 grain VLD bullets in one of my AR's, it shoots the 77 grainers well too. The 77 grain is the longest bullet that will still feed and fit in GI magazine.
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PegLeg45

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Re: AR .223/5.56 barrel twist question
« Reply #9 on: February 21, 2009, 06:01:05 pm »
Good info guys, I'm learning a lot in an area I am not as familiar with as I'd like to be.

Here is another link to a calculator for barrel twist. It was on the page m25 referenced above, but this is the straight link to it.

http://kwk.us/twist.html


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