AR-15 Gas System Identification Guide

AR rifle with brown background

There are four main lengths of AR-15 gas systems in use today: Pistol, Carbine, Mid and Rifle. Each one of these gas systems require different length gas tubes and handguards, so it is important to know which one you have before ordering parts for your AR-15.

If you don’t know what size rifle you have, it is fairly easy to determine by simply measuring the length of your handguards or gas tube.


The pistol-length gas system is used almost exclusively on AR-15 pistols or rifles with very short barrels, usually 10 inches long or less.

These systems generally have a gas tube of approximately 6.75 inches. The handguards or free-float tubes for these systems are generally between 3 to 4 inches in length.

Some people are now installing low-profile gas blocks and carbine length free-float handguards so that they can have a longer and more comfortable forearm grip area.


The carbine-length gas system, combined with barrel lengths from approximately 10 to 18 inches, is perhaps one of the most common setups across all types of AR-15 rifles.

Carbine gas tubes are approximately 9.75 inches long, which means the handguards are approximately 6.5 to 7 inches long. These include the:

  • Mako Handguard and Rails System
  • Daniel Defense Omega Quad Rail
  • Midwest Industries Carbine Forearm


Mid-length gas systems have become quite popular and are most commonly used with barrels from 14 to 20 inches long. They give you a longer sight radius and reduce some of the wear and tear on the upper receiver.

Mid-length gas tubes are approximately 11.75 inches long and require a handguard around 8.5 to 9 inches long. Choices for handguards of this length include the:

  • UTG Free-Float Mid-Length Quad-Rail
  • UTG 2-Piece Quad-Rails


Rifle-length gas tubes are generally about 15 inches long, requiring handguards ranging from 11.5 to 12 inches long.

These are usually found on rifles with barrels 20 inches in length and longer.

If you’re looking for replacement handguards for a rifle-length system, check out the:

  • UTG Free-Float Rifle Quad-Rail
  • UTG 2-Piece Quad-Rails
  • AR Carbon Fiber Free-Float Tube
  • Yankee Hill 2-Piece Quad-Rail

What size gas systems do you have on your AR-15? Are you planning to check out some of the options shared here? Let your fellow readers know by sharing in the comment section.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (25)

  1. I just purchased an AR off of my friend for my birthday and I have no idea what kind of hand guards are on the rifle. I know it’s free floating and the hand guards are approximately a foot in length. The top rail is normal Picatinny, but the bottom and sides have these very small holes that look like where small screws would go. Can anyone help me so I can buy the appropriate rails?

  2. You state that a carbine length has tube is 7”, which it is not. The tube is 9.75”, while I believe the distance to the port is 7”.

  3. My AR has a carbine length gas system with a 16 inch barrel. Is it possible for me to add a 12 inch hand guard if I use a low profile gas block? Thank you!

  4. I am not very knowledgeable on this topic but I understand rifle length gas tubes are between 15and1/8 and 15 and1/4 inch. not 13″ This is what is on my 20″ barrel and all other sights, I have looked at, who gas tubes advertise these lengths.

  5. Gas tubes are long and skinny, and mount to the shoulder thing that goes up and shoots 30caliber magazine clips in like 1/2 of a second.

  6. I’d like to buy a shotgun. There is some sort of sasquatch prowling around outside my bedroom window at night.

  7. I’ve noticed that various after-market gas blocks have different diameter gas tube ports. How do I know which size port I need to fit my gas tube? I have a carbine length.

  8. This article is WRONG. According to this the gas tube I have “is” mid length. However when mount on the barrel it only goes through the barrel nut and not the receiver. In other words, it is too short. Measured it, and it comes in at 9.5″. So to get this right, here is the following information:

    Carbine length gas tube: 9.5″
    Mid-length gas tube: 11.75″
    Rifle length gas tube: 15.125″

    FYI, all measurements are approximate.

  9. Magnificent beat ! I wish to apprentice even as you amend your website, how can i subscribe for a weblog website? The account helped me a applicable deal. I have been tiny bit familiar of this your broadcast offered vivid clear concept

  10. I posted it wrong sorry.It’s an 6x45mm 20 inch ss ar stoner HBAR.Need to know gas tube length.thanks

  11. I am building An ar 15 6x15mm upper and i purchased a 20 inch ar stoner HBAR ss from midwayusa.I need to know the length of of the gas tube to use.

  12. The gas tube lengths do not appear to be correct? A carbine length gas tube is approximately 9.75″, not 7.5″. The 7.5″ would be the approximate distance from the bore to the gas port.

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