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:
...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
fps from a 16
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 fpsTRACE 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.