Twist vs Barrel Length

What do think is more important, barrel length or twist rate in bullet weight selection

  • Twist

    Votes: 42 93.3%
  • Length

    Votes: 3 6.7%

  • Total voters
    45
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Hondo 60

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I bought a 2nd AR
1 is old school - 20" 1 in 9
The new one is an M4 knock off - a 16" barrel, also 1 in 9.

I'm curious to see which do you think is more important when selecting a bullet.
Barrel Length?? or twist rate??

And then you get to post why you think the way you do.

Please and thank you.
 
I bought a 2nd AR
1 is old school - 20" 1 in 9
The new one is an M4 knock off - a 16" barrel, also 1 in 9.

I'm curious to see which do you think is more important when selecting a bullet.
Barrel Length?? or twist rate??

And then you get to post why you think the way you do.

Please and thank you.
i'll try to find and post a link to a great article on the subject when i get home. sent to me from the late,great RC
 
Twist is more important than barrel length when designing for a particular weight bullet IMO. The longest barrel in the world isn't going to stabilize a bullet if the twist is simply too low. But there are so many variables: in theory a longer barrel will build up more velocity and be able to stabilize a bullet that's marginal with a shorter barrel, for example. As a practical matter though it seems to me barrel length has more effect on the velocity than it'll ever have on stability so I vote for twist.
 
I don't think it comes down to a single factor.

In the case of conventional bullets, for a given diameter, the heavier a bullet it, the longer it will be. The longer the bullet, the faster the twist rate must be to stabilize it. But, heavier bullets have greater inertia and so benefit from slower burning powders and slower burning powders can be more readily optimized in a longer barrel.
 
I don't think barrel length is important at all. There are 2" barreled handguns that stabilize bullets just fine. Shorter barrels tend to be stiffer all else being equal and if anything should offset any small advantage.
 
One thing I do know ...
My old AR is very accurate with 26.0 gr Varget under a 55gr FMJ at 2.225 OAL.

My new AR thinks it's OK - better than factory ammo, but ... not as accurate as the old school one.

Hence the poll question.

I realize barrel harmonics are unique to each rile or gun.
But the main difference between the 2 is the barrel length 16" vs 20".

So thanks for the replies. Please keep 'em coming
 
http://www.gunsandammo.com/ammo/pair-barrel-twist-rates-ammo/

Hopefully this will benefit someone out there. Cant recall all the specifics, just remember it was a good read...no
pictures to color unfortunately.

Oh and it was originally linked to me from the late, great, RC Model too:) Did i mention that?
I'm glad you've posted this link.
http://www.gunsandammo.com/ammo/pair-barrel-twist-rates-ammo/

Hopefully this will benefit someone out there. Cant recall all the specifics, just remember it was a good read...no pictures to color unfortunately.

Oh and it was originally linked to me from the late, great, RC Model too:) Did i mention that?
Thanks, good link. You did good Tater.
 
I vote twist for bullet grain selection but for accuracy my 20inA2 match shoots circles around my 16in carbine ,
 
One thing I do know ...
My old AR is very accurate with 26.0 gr Varget under a 55gr FMJ at 2.225 OAL.

My new AR thinks it's OK - better than factory ammo, but ... not as accurate as the old school one.

Hence the poll question.

I realize barrel harmonics are unique to each rile or gun.
But the main difference between the 2 is the barrel length 16" vs 20".

So thanks for the replies. Please keep 'em coming
The barrel length isn't the only difference, they are two totally different rifles. The barrels were made different days in different plants. The steel may be different. Pressure on the barrel may be different. Need I go on?

Barrel length will usually increase velocity but twist rate is twist rate. Maybe the velocity is too high for that bullet when shot from the longer barrel. I load a 55gr bullet with 25.0gr Varget and 26.0gr Varget and both are accurate. Maybe loading up your same load but with 25.0gr Varget instead might help accuracy in the longer barrel. It's worth a try IMO.
 
For accuracy, twist is more important. Without sufficient spin, a bullet - any bullet - will not stabilize properly. Length of barrel has no effect on stability; nor will increased velocity generate enough spin to stabilize an under stabilized bullet. The additional velocity causes a greater tendency for the bullet to yaw, or deflect (not in path, but in attitude of flight) and lose stability.

For velocity, barrel length is more important. A longer barrel - all other factors being equal - will give more velocity in normal firearms.

One more time, for those who don't know: Burn rate of powder ("faster" or "slower") depends on the ratio of bullet weight to powder charge weight. Burn rate of powder has nothing to do with barrel length.
 
Twist rate is more important for bullet selection but the powder type and charge weight needs to be tuned to the barrel length change. I've gone through the same issue with my 22" and 16" .308 rifles. I've finally found a hand load that works well in both rifles but even this load works better in the short rifle than the longer rifle (I like to use this load for hunting and the short rifle is lighter and easier to carry so I let the performance favor the short barrel).
 
In my world velocity kills and that is what my main concern is. But the shooters that want 5 bullet's in one hole, they worry a lot about RPM. The formula given to me is
MV X TWIST X 60 = RPM

mv3000 x 1:12 x 60=180,000
X 1:9 x 60 =256,000
X 1:8 x 60 = 270,000
They spin the bullet slow as possible and still stabilize it. One of them told me he couldn't care less about velocity other than he needs to know that to calculate RPM. And I quote him " If I have to I can lobe em in there like mirrors, I just need each one to fly identical to the one before it."
 
I think the twist and barrel length work together to create the spin....and each bullet has a certain rpm needed to stabilize. Short barrels will need a faster twist as they cannot generate as much speed as the longer one using the same twist rate. This is assuming you are working toward the unstable end of things....being a 1-10 long barrel will very likely be able to be as stable as a short barrel 1-9, but of course how they're loaded and the particular bullet probably more important. If you stay away from the extremely long bullets for the twist rate it probably doesn't make a whole lot of difference.
 
I didn't vote, IMO, it's a combination of both.
That said, I really prefer the longer bull barrels,
with the 1:7 twist. They just work better, across
the entire range of bullet weights, for me. It
may just be what I am accustomed to. YMMV.
 
Twist rate. I can compensate for POI due to barrel length/dwell time/MV/muzzle jump, but I can’t compensate for lack of stability. ymmv
 
I've been reading lately about these guys shooting 30 BR. I got curious when I saw .308 barrels with a 1:14 twist and wondered who would want that when everything seems to be moving to 1:10

So I guess my answer is faster twist isn't always better
 
Hondo60 wrote:
I'm curious to see which do you think is more important when selecting a bullet.
Barrel Length?? or twist rate??

One thing I do know ...
My old AR is very accurate with 26.0 gr Varget under a 55gr FMJ at 2.225 OAL.
My new AR thinks it's OK - better than factory ammo, but ... not as accurate as the old school one.
Hence the poll question.

But the question you asked in the poll would not elicit an explanation for the difference. For any given diameter, the heavier the bullet of any particular configuration is, the longer it will be. The longer the bullet, the more it has to spin in order to remain stable in flight. The spin rate is determined by the twist rate so twist rate will always control what bullets can be accurately fired through a given barrel.

A barrel's length determines 1) how long the rifling has to work on the bullet, and 2) how long the pressure generated by the propellant gasses have to accelerate the bullet. In your case, the 20 inch barrel is 25% longer than the 16 inch and work on and accelerates the bullet for a longer period of time. If you fire the same powder charge and bullet combination in a shorter barrel, you will get a lower velocity * and the difference in velocity alone could explain a difference in accuracy.

-------
* It is possible to create a loading where the powder has burned completely and the gas pressure is no longer building as the bullet exits the shorter barrel. The bullet in the longer barrel would have to overcome the friction of the additional length resulting in the bullet exiting the longer barrel with a lower velocity than the bullet coming from the shorter barrel. But this is something that must be done as a "parlor trick" and is not the norm.
 
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One thing I don't think I added was both are twisted 1:9.
The ammo was worked up in the original (to me) old school AR15A1 with a 20" barrel.
I suppose I should rework the ammo in the newer 16" barrel. (again 1:9)

I posted this way back in Nov 2016.
Since then I'm thinkin' It's just different guns react differently to very similar ammo.

Got a pic of the old school
ar15a1.jpg
 
Yep twist rate. It has been explained quite well already. Yes, no two rifles will shoot the same ammo exactly to the same POI. I have yet to see two that are so close I could not see a difference with ammo tuned to one rifle. Unfortunately you really do need to work up a load for the other rifle if you want it to shoot it's best.
 
39 to 3 favoring twist when I voted (twist, of course).

I shoot several short barrels, and all of them can be made to shoot incredibly well. Alternatively, I've never had luck getting an "under twisted" barrel to shoot well. It's really easy to make up for barrel length with twist. Not as easy to make up for a "twist-too-slow" by adding barrel length.
 
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