AR-15 Gas Tubes: Which Length is Best?

The AR-15 gas system's length is arguably one of its most important specifications -- and, unlike so many other AR components, there are, in fact, "best" gas lengths for each caliber and barrel length. 

Why Gas System Length Matters

The AR-15's driven by direct impingement. That means the gas expelled by the cartridge directly drives the bolt carrier group to cycle the action, with no gas piston necessary. This direct impingement drive means the length of the gas system -- specifically the tube that carries gas from the barrel to the upper receiver -- plays a key role in determining whether your AR operates reliably. 

If a Gas Tube is Too Short

If your gas tube's too short, your AR-15 will be "over-gassed." That means too much gas is entering the upper receiver and imparting too much energy on the bolt carrier group. This causes excess felt recoil, and it wears down the seals on the bolt inside the carrier.

This excess gas pressure also expels additional carbon fouling into both the upper and lower receivers, which can gunk up the bolt carrier group and fire control components. You may even experience blowback -- hot gas directly blasted into the face through the ejection port. Not good.

If a Gas Tube is Too Long

A gas tube that's too long is even worse. In this case, not enough gas will make it to the bolt carrier group. This causes the bolt carrier group to short stroke, which contributes to failures to eject and feed. You may also experience jams, and the bolt may not be caught by the bolt catch once the last round leaves the magazine.

Gas Tube Length: All About Dwell Time

Gas tube length is determine by one measurement: Dwell time. Dwell time measures the length of time a bullet remains in the barrel after ignition and before it exits the muzzle. 

A subset of dwell time measures the time between the bullet passing the gas port and exiting the muzzle. This "post-gas port" dwell time is what forces gas to enter the tube while under pressure, so it can actuate the bolt carrier group.

How Gas Tube Length Affects Dwell Time

The more time a bullet spends between the gas port and muzzle, the more gas is forced through the port and tube, back into the upper receiver. In other words, the longer the dwell time, the greater gas pressure. This causes the bolt to cycle with greater force.

The moment the bullet exits the muzzle, the gas in the bore seeks the path of least resistance. It rapidly loses pressure and exits the muzzle, rather than traveling through the gas tube.

That's why gas tube length is inverse to gas pressure: A shorter tube increases dwell time, providing more time for gas to travel through the tube before the bullet exits the muzzle. The shorter tube also allows the gas to retain more heat, and thus, more pressure.

So, What's The Best Gas Tube Length?

For 5.56 NATO and .223 Remington-chambered AR-15s, all the research on dwell time has been done to find the best gas tube lengths for various barrel lengths:

• 18"+ Barrel: Rifle-length gas tube
• 14.5" to 18" Barrel: Mid-length gas tube
• 10" to 14.5" Barrel: Carbine-length gas tube
• < 10" Barrel: Pistol-length gas tube

About Those 16" Barrels With Carbine Gas

You could make a wild guess that up to 33% of all 16"-barreled AR-15s come with a carbine gas tube, and you might be right. But this configuration doesn't seem optimal. It flies in the face of the recommendations above. What gives?

The carbine gas tube was (and still is) so popular because the U.S. military runs carbine tubes on its M4 rifles. But, after doing extensive testing, the military found mid-length gas tubes are superior for 14.5" and longer barrels.

They found the mid-length tube "significantly extended the life of the weapon system," while also improving reliability. That's right -- the longer tube, while forcing less gas into the upper receiver and bolt carrier group, actually experienced fewer malfunctions. Win-win. You can read the study here.

But, to preempt the question: No, there is nothing wrong with running a carbine gas tube on a 16" upper. It's a proven setup that just so happens to produce more felt recoil, and perhaps a little extra fouling in the receivers.

Many shooters prefer the carbine tube on a 16" barrel, because it allows for the use of a shorter and lighter handguard. And some barrel manufacturers drill smaller gas port holes in their barrels, which allows a lesser amount of gas to flow through the tube. In this case, a carbine gas tube may, in fact, be appropriate.

Optimal Gas Lengths for Other Cartridges

Other popular AR cartridges require specific gas system lengths to perform optimally with particular barrel lengths. Here are the best gas system lengths for other cartridges by barrel length.

6.5 Grendel Gas Tube Lengths

• 18"+ Barrel: Rifle-length
• 16" - 18" Barrel: Mid-length

300 Blackout Gas Tube Length

• All Barrel Lengths: Pistol-length

No matter the barrel length, a pistol gas tube is required to cycle subsonic 300 BLK loads.

.308 Winchester Gas Tube Length

• All Barrel Lengths: Rifle-length

The .308 cartridge is considered "over-gassed" in virtually all AR-15 barrel lengths. So, even with a short (16") barrel, the short dwell time provided by the rifle-length gas tube is optimal.

(Adjustable Gas Blocks Are Great, Too)

Even with the optimal gas tube for your cartridge and barrel length, you might run into issues with what seems to be an over-gassed or under-gassed setup. This could be caused by a buffer that's too light or heavy, or a gas port diameter that's a little out of spec.

In either case, an adjustable gas block is a great investment for any AR-15, no matter the caliber -- and it's especially warranted if you chamber a mixture of supersonic and subsonic loads (like 300 Blackout), which produce vastly different gas pressures.

Ready to install or swap your gas system? Read our next guide: How to Build an AR-15 Upper.