Author Topic: Long Range AR - Barrel Length?  (Read 3422 times)

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Frisco Pete

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Long Range AR - Barrel Length?
« on: May 22, 2012, 03:58:14 PM »
There is a lot of talk about whether or not you need a 20" or even a 24" barrel for a "long range" AR15, as compared to the popular 16 inch AR carbine length.  While there are a lot of factors that go into this choice, many feel that the higher velocity of the 20" barrel makes the bullet shoot farther. 
Just as a recent example, cmchap76 evidently was thinking along these lines when he was deciding on scoping his AR.  He said:
Quote
...Make this particular rifle a long range shooter since it has the 20in barrel and go with a more standard rifle scope.

So just how much farther and flatter shooting advantage does that extra 4 inches of barrel length give you?

I ran the numbers through my Oehler Ballistic Explorer program based on actual chronograph results using both a 16" and a 20" barrel AR chronographed with factory Lake City M193 55-gr FMJ Ball ammo (as sold as both American Eagle and Winchester).  First of all, this ammo is very uniform in the velocities it gives.  Shot-to-shot velocity spread or deviation is very low.  It is also pretty hot ammo and pretty much lives up to factory claims for velocity.

In my testing M193 Ball ammo chronographed an average of:
3240 fps from a 20 inch AR15
3110 fps from a 16 inch AR15
giving a 130 fps increase/drop in velocity for that 4 inches of barrel difference, or around 32.5 fps per inch of barrel.

For our purposes, we will consider up to 500 yards "long range" for this intermediate round that was designed around a 300 meter limit.  We will also zero both rifles evenly at 200 yards, which is a very good zero range for the .223 cartridge.  Scope or sight height above the bore is 3.2 inches - common for scoped ARs.

Plugging these numbers in the Ballistic Program with this ammo we get this comparison graph:
TRACE 1 is 20 inch 3240 fps
TRACE 2 is 16 inch 3110 fps



We can see that there is virtually no discernible difference in trajectory until the bullets go beyond 250 yards, then the lower velocity of the 16" drops more.

So what is the difference in drop in numbers?
At 300 yards, the 16" is only 0.61" lower.
At 350 yards, the 16" is 1.11" lower
At 400 yards, the 16" is 1.76" lower
At 450 yards, the 16" is 2.60" lower
At 500 yards, the 16" is 3.63" lower
At 600 yards, which is a match range distance but not on the graph, the 16" is 6.39" lower.

Something to think about is that a person can't really hold the rifle and scope crosshairs super accurately on the target at the ranges where the drop is most significant.  This would be aiming error.  It would be enough, especially at under 450 yards, that a shooter could not really tell which barrel length had the flattest trajectory.
Another thing to consider is that on known ranges a shooter can compensate for minor variations in trajectory such as these.

Other factors come into play as well.  A shorter barrel of the same diameter is stiffer than a longer barrel and is less affected by barrel whip and vibrations.  Longer barrels need to be a bit thicker/heavier to compensate for this.  That is why Bench Rest competitors often just use a 20" barrel or so.  This is also the reason why many of us have come to appreciate the accuracy potential of the 16 or 18 inch AR barrels.  If I recall correctly, Onepoint has gone away from 20" AR barrels for his long-range prairie dog hunting because the longer barrel gives him no discernible advantage, and isn't quite as handy.  He loves the accuracy of the 16", and the slight loss of trajectory is a non-issue because of the reasons stated above.
When coyote/predator hunter Fred Eichler designed his signature Predator series AR15 for Rock River, it sports a full-diameter 16-inch barrel that is both stiff and accurate (3/4" MOA), and handy to swing onto a target, and less likely to hang up on bushes during that swing - unlike some of the heavy 20-24 inch target barrels that have all the dynamic handling qualities of a truck axle.

Obviously there is nothing wrong with a 20-inch tube, and you will see them on target competition AR15s - starting with the HBar profile and going to Bull - and also ARs that don't need to be carried a lot.  Obviously, there is no disadvantage as far as trajectory goes - and many feel that every tiny edge is important.

One area where every fps of velocity may give an advantage is in proper bullet performance, particularly when it comes to military FMJ ammo.  This non-expanding bullet type relies on velocity to make it yaw and break up upon contact with flesh.  When 55-gr M193 ammo was used in 20" M16s, this was less of a problem.  However when the switch mas made to "penetrator" 62-gr M855 ammo - and then another change was made to use the 14.5" barrel M4 - this all conspired to make ammo that has a lower velocity to be shot in a carbine that lowers velocity as well.  The result was that at almost any longer range, the M855 would "icepick" and fail to yaw and break up when it encountered flesh.  Of course civilian ammo/bullets are often expanding tip bullets and are far and away less sensitive to velocity issues, and will expand at ranges that exceed that of even the M193 bullet.
Of course only you can decide how this affects your shooting and intended usage of your civilian AR15.

But a shooter trying to decide on which AR barrel length is best for him needs some solid information on the long range effect/advantage, or not, of the two most popular lengths.  And that information is here. 
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xICEMANx

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Re: Long Range AR - Barrel Length?
« Reply #1 on: May 22, 2012, 04:38:53 PM »
Thanks for the writeup. Alot of good data in there. I think I'll stick to my 16" barrel. I also hunt varmint and was thinking of going to a longer barrel. Now I don't think it's neccessary, plus the carbine length is lighter.

tungsten74

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Re: Long Range AR - Barrel Length?
« Reply #2 on: May 22, 2012, 05:16:44 PM »
Iv looked at both wonderig this exact thing. Now thinking 450yards with almost no difference beyond my abilities to hold. 16 you can conceal better then the 20. For out past that I'd want a bolt action and larger round. This is a very very useful tool and 16 inch are usually cheaper and apparently as accurate. Thank you!
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bow_breaker

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Re: Long Range AR - Barrel Length?
« Reply #3 on: May 22, 2012, 05:25:49 PM »
Nice write up. A lot of this was the reasoning of my 18 inch HBAR.
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Onepoint

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Re: Long Range AR - Barrel Length?
« Reply #4 on: May 23, 2012, 12:56:46 AM »
I have a couple 20" uppers, but not because I wanted the edge in velocity, one is a FN lightweight SP1, I keep it because it excells in irons sight shooting, being easy to handle and yet have long sight radius.   The other is a Colt govt profile 1/7 twist barrel that I came across in a trade and built into an upper.

I have one 18" upper.  A Douglas stainless fluted Hbar 1/7, thinking it would be the ultimate walking varminter, and it is accurate and manageable to handle, but guess what, I end up varminting most with a mid weight fluted 16" Saber Defense barreled upper.  It may not turn in quite the tight groups, but the coyotes, foxes and prairie dogs don't seem to survive all that much to complain over the 18", especially at the ranges I shoot most.  I still keep the 18" around for when I really want to stretch it out.

Aside from all that, if RR ever sells those Fred Eichler forearms separate, I am all over one. :)

Great FAQ material Frisco.
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trailblazer

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Re: Long Range AR - Barrel Length?
« Reply #5 on: May 23, 2012, 09:34:21 AM »
I'm still in the planning stages for my first AR build. It will be a 16" barrel. Is there much difference in accuracy between an M4 profile compaired to a fatter  barrel?
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nightgunner

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Re: Long Range AR - Barrel Length?
« Reply #6 on: May 23, 2012, 09:43:09 AM »
I'm still in the planning stages for my first AR build. It will be a 16" barrel. Is there much difference in accuracy between an M4 profile compaired to a fatter  barrel?

There should be no noticeable difference. the M4 profile was created to aid in the mounting of the M203 Grenade Launcher. a true heavy barrel will dissipate heat better, but that really only matters if running F/A.

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Onepoint

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Re: Long Range AR - Barrel Length?
« Reply #7 on: May 23, 2012, 10:10:14 AM »
Heavier barrels flex less and help with recoil because of weight.  But mostly the care taken in boring and rifling will impact the barrel performance more than anything.   And unless you are shooting match grade ammo in a match grade barrel, you will likely not notice much difference anyway.   You will notice a big difference in balance and handling in different barrel weights though.
"War is not 'the best way of settling differences; it is the only way of preventing them being settled for you." - G.K.Chesterson /   "Reality is that which, when you refuse to believe, or stop believing in it, does not go away."

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