The most common barrel length of an AR-15 is 16 inches, and it’s not close. There are a bunch of alternative lengths, some common (pistol or SBR length barrels), others become a little less. But the 16-inch barrel is still and probably always will be the most popular. Why? It’s not because it is the optimal barrel length for the 5.56x45mm; no, shorter may be better as the military-issue M4 standard is 14.5 inches.
But more on that later. No, 16-inch and longer barrels are popular because of regulations. It is all about staying out of jail. Or, to be honest, most of us would love an SBR but we have no desire to get the tax stamp.
So, let’s take a look at the popular barrel lengths at 16-inches and up.
What’s Up With 16 Inches?
Like I said earlier, if we’re being honest the 16-inch flavor only exists en masse to satisfy the feds. Nobody is really all that excited about a standard 16-inch barrel. Also, most sensible folks don’t want to do prison time for having a barrel that’s too short.
A quick caveat: you can use a 14.5” barrel legally on your AR-15, so long as the flash hider or muzzle device is permanently pinned and welded into place and creates a total length of 16 inches from breach to tip. This would also be the most accurate depiction of an M4 clone that you can own if you’re into that sort of thing.
Is a 16-inch barrel a bad thing? No. But we’ll get into the specifics of 16-inch barrel performance later.
The National Firearms Act of 1934
We’re heard a lot of talk about the NFA, but what exactly does it say? Well, here is the ATF’s summary of the NFA. Also, here is the NFA handbook, good for some light reading. According to the ATF, it is mainly for people in the business of “importing, manufacturing, and dealing in firearms defined by the National Firearms Act.”
The original NFA was enacted in 1934, largely as a response to organized, violent street crime. Did it work? Hell yeah it did! When was the last time you heard about violent crime in a major city? I’ll wait.
For practical purposes, though, the NFA means that you can’t have a can, an SBR, or a pistol brace without a tax stamp
We’re back to the simple but effective 16-incher. There was a time (like, three years ago) when you could pick up a simple 16-inch barrel on sale for about $60. Super common, cheap, and easy to work with.
For the average guy who isn’t Keanu, a standard AR-15 16-inch barrel is totally adequate. An AR-15 carbine with this barrel length is still an ultra-compact weapon system. At only a hair shy of three feet long, it is still nimble and maneuverable.
Using the timeless M193 cartridge, a 55-grain full metal jacket projectile in the boattail composition, a 16-inch barrel is a good compromise between muzzle velocity, while keeping the bore pressure at a reasonable level. What does bore pressure have to do with anything?
There is an exponential increase in bore pressure as the barrel length decreases. This is the act of the bullet “uncorking” as it exits the barrel. It is why a snub-nosed revolver sounds so much louder than a four-inch service revolver. The bore pressure with a 10.5-inch barrel is over double that of a 20-inch barrel. Increased bore pressure results in much higher levels of sound, flash, and significantly higher strain on suppressors.
To summarize: a 16-inch barrel is the best compromise of muzzle energy, muzzle velocity, bore pressure, long-distance performance, and maneuverability/compactness without getting a tax stamp.
Eighteen Inches: The Prototypical Mk 12 Special Purpose Rifle
About the only good thing you can say about any war, but especially the GWOT is that some cool stuff is produced as a result. At the beginning of the OIF/OEF, the M16A2 was still the standard rifle in most armories, iron sights, and all. What’s an optic? In fact, I didn’t qualify on the M4 until around 2010 (non-combat arms branches were the last to upgrade).
Throughout the course of these wars, the DoD produced a bunch of variants for different missions, and in terms of cool factor, the HK Mk 12 Special Purpose Rifle (SPR) is at least a mid-weight contender.
The 18-inch barrel is a full 3.5 inches longer than the M4, intended to increase the effective range of the M4-pattern rifle while keeping the overall length shorter than an M16A2/A4. The DoD took pieces here and pieces there from other projects and threw on a free-floating handguard and a KAC two-stage trigger.
Using M855 ammunition as a baseline, the 18-inch barrel produces only an additional 100ft/s or so over the 16-inch AR-15 barrel. The long-range improvement of adding two inches is fairly negligible.
It’s 1985 All Over Again: 20 Inch Barrels and A2 Buttstocks
As a product of the 80s, I enlisted in the Air Force in May 2002. We qualified with M16A2s, which was my first exposure to any Stoner AR pattern design.
A 20-inch barrel is excessive with modern ammunition technology. It is a design from a bygone era. Barrel length is not as closely related to accuracy as conventional wisdom suggests’ barrel whip does occur in longer barrels, although unless you are a precision match shooter, you’ll probably never notice it. For weekend shooters, there is no advantage to a 20-inch barrel except that the bore pressure continues to drop with barrel length.
The bore pressure of a 20-inch barrel is roughly 25 percent less than a 16-inch barrel, so the noise and muzzle blast are significantly less.
However…if you are a sucker for nostalgia like me, then you should definitely keep an A2 clone in your arsenal. I mean, there isn’t really a downside.
There is no ballistic reason to have a barrel longer than 20 inches. Or even 20 inches to be honest. But longer than that doesn’t make any sense.
Can you do it anyway? Hell yeah, you can! If you want to be super cool, throw a 24-inch barrel on there with a standard M4 collapsing stock. That looks super cool.
Modern propellants and projectile technology have been made to optimize the 5.56x45mm for shorter barrel lengths. It is a compact weapon system caliber. However, since the feds are making braced pistols essentially an SBR, the only way to practically avoid tax stamps is a 16-inch barrel or longer. A 16-inch barrel is fine for about 99 percent of all shooters; anything longer is purely aesthetics. But if you want an SPR or A2 clone, go for it!
We carry a full line of parts to build whatever you want, whatever length you want to build. Want an SBR? We got it. Standard 16-incher? Yeah, that too. Also, make sure to check out the LP-1 Promethean optic while you’re there!