AR-15 Barrel Lengths, Explained – The Shooter’s Log

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Some people prefer barrel lengths around 30 inches when it comes to their AR-15s. Others like 12.7 inches. There are even a few crazy people out there that desire five inches.

We do live in America, so in theory, they all have the option to spend their money on the length they like.

The great reality is each of these shooters may well be right. We really have to look at their specific-use profile to see why they choose what they choose.

Let’s dive deeper into the differences between AR-15 barrel lengths.

Longer AR-15 Barrel Lengths

On the longer end of things, I run a 24-inch barrel White Oaks Armament upper on my precision-shooting AR.

With that rifle, due to barrel length and some other things, I can run a bit more pressure than I can in my carbine-length options.

The extra length, as well as the added pressure, provides my 77-grain OTM Bergers with a velocity of about 2,900 fps.

This is quite a bit lower than just over 3300 for some 55-grain loads, but a lot higher than it would be from a shorter barrel.

The much better ballistic coefficient of the 77-grain bullet makes for much better precision out past 500 yards and carries a bit more energy.

Shooters who are determined to go past 750 yards with an AR-15, often run barrels up to 30 inches. They can place the gas port even further towards the muzzle.

This reduces recoil with higher-pressure rounds, and makes for slower bolt movement as well. This helps to keep the lower and lower parts from experiencing accelerated wear.

The extra length aids in gaining higher velocities through complete powder burn and (if coupled with a heavy contour) also aids in greater precision through less barrel whip and maintaining a cooler barrel temperature.

These shooters also tend to single load 80 to 90-grain bullets, but that is another topic. Others running wildcat calibers like 6mm Fat Rat also tend towards the longer barrels.

American Flag AR-15
This AR-15 offers a good balance between length and portability.

My 3-Gun barrel is a 20-inch. This is a compromise between the 24 inch of my precision gun and my carbines. The purpose is to provide greater velocities for the distance shoots.

Some of the stages incorporate 200 to 300-yard shots. With this rifle, the Frontier (Hornady) 55-grain FMJ achieves just under 3200 fps. In my carbine, I get just about 2940 fps.

The extra velocity provides a better maximum point-blank range and I often don’t bother to adjust my optic from 1.5 power for the first long-distance shots.

I use a 30-yard zero for 3-Gun. This provides hits that are slightly low (less than one inch) on anything closer than 30 yards, but most of those are silhouette targets that are getting a double-tap and have larger than a 12-inch success window.

The 30-yard (near) zero crosses back to a far zero at approximately 200 yards. With the carbine, I am never more than 2.2 inches high or low from zero to 250 yards.

Out at 300 yards, the bullet is 5.6-inches low. Simply changing to the 20-inch barrel keeps my shots at +/- 2.6 inches all the way out to 300 yards.

Long-Range AR-15
AR-15s with longer barrel lengths are great for long-range shooting.

Shorter AR-15 Barrel Lengths

The guys who claim the 12.7-inch barrel is the best are delusional in my opinion. However, for short-distance/close-quarters work, they may have a point. This is especially true if running a can.

If we assume a 55-grain bullet at 3300 fps from a 24-inch barrel, we will see roughly 2700 fps from the much shorter barrel. For close work, we are not concerned with drop.

It is however a concern with energy. The 3200 fps bullet has 1275 ft/lbs of energy at the muzzle and 1170 ft/lbs at 50 yards. The 2700 fps bullet has 890 ft/lbs and 830 ft/lbs respectively.

You also have to be concerned if the lower velocity will allow for soft-point expansion or to initiate tumbling.

To put that in perspective, you have lost the entire energy of a 9mm Luger bullet at point-blank. Or, the shorter barrel’s bullet is equal to a hit at 275 yards from the 24-inch barrel.

Another perspective to this is most state hunting agencies that specify muzzle energy for deer hunting desire 1000 ft/lbs.

This trade-off may make sense when small footprint, stealth and relative quiet are the overriding desires.

9mm AR and .300 Blackout AR
AR-15s with shorter barrel lengths are best suited to calibers other than 5.56 NAT0, such as 9mm Luger and .300 AAC Blackout.

I would go even shorter in that scenario, but I would happily leave the 5.56 NATO cartridge behind. That is exactly what I did with my 8-inch .300 AAC Blackout.

The cartridge is designed to run on shorter barrel lengths with full powder burn happening by 10 inches. This is my bedside AR. I run 190-grain Maker Bullets at 1060 fps.

My zero is at 20 yards. I am about 1.5-inches low at contact distance. The bullets are very close to dead-on at both 25 ( 0.2-inches high) and 50 yards ( 0.2-inches low).

The trade-off I make for short and whisper-quiet is a lack of energy. At the muzzle 475 ft/lbs is generated, dropping to 460 ft/lbs at 50 yards.

However, you have to remember this is a bullet that starts at 0.308 diameter and achieves no less than 50% expansion down to 900 fps (over 500 yards).

Both the expansion and much lower energy will decrease penetration. This is a good thing for home defense, as the 12 to 16 inches this round achieves is still a through and through on most unarmored people.

It also means all the energy is transferred through a ragged hole at least 0.45 inches (possibly as much as 0.70 inches) in diameter.

The 5.56 bullets start out at 0.224 inches and will not open to anything resembling 0.45 inches.

They may tumble and/or expand (0.275-0.300 inches), but both expansion and tumbling are much more consistent with higher-velocity impacts.

Another less quiet option with the .300 ACC Blackout is to run 125-grain supers. These generate 1,300+ ft/lbs of energy at contact distance and are a much better option than my subsonic choice out past 50 yards.

Assuming a 30-yard zero, these rounds should stay at +/- one inch out to 150 yards. I would still run the can as it will greatly reduce pressure and volume, but they will not be hearing safe, even outdoors.

Those who want to run five-inch barrels on their AR are in one of two camps. The first being people who really enjoy fire-spewing, super-loud AR’s sending low-velocity bullets downrange.

If it makes them smile, more power to them. The others are people using pistol-caliber rounds in an AR platform.

For those running super-shorty 5.56 NATO AR’s, the five-inch barrel gets about 1800 fps with the same rounds the 20-inch achieves 3200 fps with.

These rounds also are right at 400 ft/lbs of energy with no likelihood of expansion or tumbling. With the lack of mass, their penetration will likely not hit the FBI’s desired 12 inches.

The exact same barrel length in a 9mm Luger AR, likely gives better performance than your carry piece and is definitely easier to make rapid hits with.

We know you are running an AR pistol brace for easy sight alignment and we all have a Holosun or Swampfox red dot mounted to the forward rail.

Those 124-grain JHP rounds are exiting the barrel at somewhere between 1,100 and 1,250 fps and are providing up to 500 ft/lbs of energy and expansion up to 0.70 inches.

.45 ACP AR-15
This .45 ACP has a shorter barrel length with a pinned and welded flash hider to comply with minimum rifle overall length requirements.

Conclusion

We can certainly see that my preference is towards longer barrels. My 9mm Luger AR’s have nine-inch and 16-inch barrels. My .45 ACP has a 12-inch barrel (plus 4.5-inch welded flash hider).

The shortest 5.56 NATO is a 16-inch and I don’t see any likelihood of changing that.

But, I enjoy shooting longer distances and I don’t see much upside to the slow 5.56 bullets when there are perfectly good .300 Blackout or pistol rounds that are often cheaper and perform better.

But it is a preference. Some people like blondes, I don’t.  I am not wrong, just different. Just like with dating preferences, it is personal and very few of us are truly wrong so long as everyone is having fun.

What are your favorite barrel lengths for the AR-15? Why? Let us know in the comments below.



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