The Length Of An AR-15’s Barrel Is A Topic Of Discussion For Some People. How Long Should It Be? Are There Any Benefits For A Particular Barrel Length? How Does The Barrel Length Impact My Rifle?
We’re going to answer some of these questions while exploring the nuances of barrel length.
Legal Restrictions
There are certain restrictions put on how short a rifle barrel can be in the US. The minimum is 16 inches. This means a pin and welded barrel that reaches 16 inches, a barrel that is 16 inches without a muzzle device, or a barrel longer than 16 inches does not need to have a Form 1 Tax Stamp from the ATF.
Barrels that are below 16 inches even with a pin and welded muzzle device require a tax stamp to be legally owned. This does not cover things like AR pistols, but that extends beyond our scope for this article. Always check federal, state, and local regulations before purchasing a firearm or firearm part.
Performance
The rifle’s performance is generally what people focus on and is what really matters beyond legality. Assuming all legal requirements are followed you have several options for your AR-15’s barrel.
10.5 inch barrel
A 10.5 inch barrel is the lowest standard length you should choose for a 5.56 AR-15. Any shorter and the velocity drop off is too great for it to be a viable option. It also will require the gun to be tuned in order to cycle consistently.
The shorter amount of barrel needs consistency in terms of being suppressed or unsuppressed for the best results. It is much harder to make a 10.5 inch gun that works both ways due to the back pressure of most suppressor designs.
11.5 inch barrel
While it is only an inch longer than the shortest viable barrel length, 11.5 inch guns are more reliable than 10.5 guns. This is because of the fractionally longer time for the propellant to burn and other highly technical factors, like dwell time, that make the gun work.
If you are going to go the short barreled rifle route (SBR), the 11.5 is going to be your best option for 5.56 loadings.
13.7 inch inch barrel
This barrel length is a newer offering on the market. It’s shorter and more maneuverable than longer barrels but it can still be pin and welded to meet the 16 inch requirement. It can best be portrayed as the average rifle barrel. Not too long or heavy but still provides good velocity and stability for 5.56 to be effective.
If you’re looking for a more general use rifle barrel, the 13.7 is a good option. This is because it is designed to fall directly in the middle of all barrel lengths for performance, length, and maneuverability.
14.5 inch inch barrel
The 14.5 barrel is the standard length for the US M4 and was a staple of M4 clones. It offers a good compromise on length and overall performance. Most 14.5 guns will run reliably without too much tuning, but won’t provide the velocity of the longer barrel lengths.
Using a 14.5 barrel usually will require using very long muzzle devices to meet the federal minimum. This can be problematic if you don’t get the right length muzzle device to pin and weld. And if you were to SBR the rifle, it would be better to get a shorter barrel length for the hassle.
16 inch inch barrel
This is the base length of AR-15s on the civilian market. It offers relatively similar velocities to 20 inch barrels without the necessity to pin and weld a muzzle device in order to be legally acceptable.
Some people may find the 16 inch barrel and muzzle device a little unwieldy, but it is a perfectly capable rifle for the average individual.
20 inch barrel
Most early load options for 5.56 are optimized for use in a 20 inch barrel. Loadings like M193 and other 55 grain options will perform better out of 20 inch barrels while still being more affordable than heavier projectiles.
A 20 inch barrel is starting to become a more niche option, and is particularly popular for M16 clones.
Twist Rate
The twist rate of your barrel will have a significant impact on your gun’s performance. There are three major variations of twist rate from fastest to slowest.
1:7 twist
This twist rate is the best for heavier bullets. If you use 70 grains or more this is the twist rate for you. However it will over-stabilize light bullets. This can result in lowered accuracy with lighter bullet loads.
1:8 twist
This is the middle ground if you plan on using a wide variety of 5.56 loadings. A 1:8 twist rate will stabilize 55 grain loads all the way up to 77 grains and similar heavy projectiles. If you’re looking to get the most versatile rifle, a 1:8 twist has the best balance of factors most loadings.
1:9 twist
This twist rate is best for light loads. If you shoot predominately 55-60 grain loads, you will get good performance out of a 1:9 twist barrel. It’s just enough to stabilize the round which allows for better yawing effects.
What’s The Best Option?
If you are looking for a good rifle that can handle most loads and has the least hassle. A 16 inch barrel with a 1:8 twist rate is what you’re looking for. A close second is a pin and welded 14.5 with a 1:8 twist. These balance length and performance the best
If you want to maximize any load, a 20 inch barrel with a 1:8 twist will have great performance at the cost of overall length. The 11.5 with a 1:7 twist will be better for your shortest options, since you should be using heavy loads to make up for the lower velocity.
At the end of the day, barrel length and twist rate comes down to preference. As long as the combination performs enough for your needs, it’s a good choice.