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Selecting An AR-15 Handguard Part 1: Gas Systems

Selecting An AR-15 Handguard
Part 1: Gas Systems
By Roy Hill, Brownells Copywriter

AR-15 handguards come in a vast variety of sizes and styles that perform different functions.
 
The AR-15 rifle is one of the most popular firearms in the United States for several reasons. One of them is that its modular design allows just about anybody to swap out parts, install accessories, or customize the rifle. The rifle's forend – usually called the "handguard" in AR-Speak - is one part that's easy to change. By getting a different handguard, you can improve your rifle's comfort, give it more accessory-mounting versatility, or even enhance its accuracy potential.

Handguards & Gas Systems

There are different ways to classify and categorize AR-15 handguards: free-float or non-free-float, the number of accessory rails or the lack of rails, the materials they are made of, or how they attach to the rifle. Some handguards simultaneously fit into different categories. However, the overall length of the handguard is usually determined by the length of the gas system on your rifle. A handguard needs to be long enough to reach from the front of the upper receiver to the gas port on the barrel.

Top to bottom: Rifle, Mid-Length, and Carbine military-type handguards.
 
The most common handguard lengths are rifle, mid-length, carbine, and pistol. They correspond to gas systems with the same names. Rifle gas systems have gas tubes that are usually about 13" long, and are often found on guns with barrels 20" long and longer. Rifles with the classic looks of the military M16A1 or M16A2 employ rifle-length G.I.-style handguards that extend to just behind the gas system – smooth-sided triangular handguards on the M16A1 and the tapered round handguard with ribs for extra grip on the M16A2.

Classic A2-style (carry handle) upper receiver from DPMS with a standard Rifle-length gas system.
 
The carbine-length gas system is the other "official" AR-15 gas system, in that it derives from the M4 carbine used by the U.S. military. It's about 7-1/2" long and works great on the short-barreled rifles common to military and law-enforcement applications, like the true M4 carbine. While the actual M4 barrel length is 14.5", civilian rifles are required by Federal law to have barrels at least 16" long. Even though a carbine gas-system can cause high pressures in such short barrels, it is the most popular gas system. About the only time anyone actually has problems with the higher pressures of a carbine-length system on a 16" barrel is when using a suppressor. Because carbines are so popular, carbine-length handguards are available in an almost endless set of choices.

A High Standard flattop upper receiver with Carbine gas system and handguard.
 
Mid-length gas systems are usually 9-1/2" long and were designed to improve reliability and reduce system wear on ARs with 16" or longer barrels. The longer gas system shortens "dwell time" – the amount of time a bullet stays in the barrel once it has passed the gas port. The longer the dwell time, the greater the rearward pressure that builds up. Repeated exposure to really high pressures can cause parts to wear more quickly. The mid-length gas system puts the gas port closer to the muzzle on a 16" or 18" barrel than on a carbine gas system, so it reduces dwell time and, consequently, wear and tear on the gas system. Another benefit is that the longer handguard gives the shooter a larger grasping area and more room to mount accessories.

Syrac Ordnance flattop upper with Mid-Length gas system and a
handguard that covers the low-profile gas block.

 
Pistol gas systems are usually around 4-1/2" long and found on AR-15 style pistols. Although it's the least common length for a gas system, more and more handguards are being produced for it, as more shoooters try AR-15 pistols and like them. Some folks install carbine-length handguards to AR-15 pistols to give more gripping surface for better control when firing.

Adams Arms gas piston upper with a Pistol-length gas system.
 

Correct Handguard Length

Generally, you need to match the handguard length to the gas system. If you use a standard gas block, the only handguard that will fit on your AR-15 is the one that matches the gas system length. But if you use a low-profile gas block, it is possible to put a longer handguard on a shorter gas system - such as a carbine handguard on a pistol gas system, or a mid-length handguard on a carbine gas system. Some shooters like the longer gripping area. Others just like the look it gives to the firearm. Both are good reasons to add a longer handguard.

Just don't put a shorter handguard on a longer gas system, like a carbine handguard on a rifle gas system. Such a set up leaves the gas tube exposed and easily bent or damaged. Also, realize that using a low-profile gas block usually means giving up the traditional sight tower of the G.I. gas block, so you'll have to get a gas block with a small Picatinny rail on top and add a flip-up sight or dedicate the rifle to using some sort of optic.

Even though gas-system length typically determines handguard length, there are some handguards purposely designed to be a bit different. One example is Yankee Hill Machine's Specter  handguard designed just a little longer than their mid-length handguard. The Specter, when combined with a YHM low-profile gas block, will cover up the gas system on both carbine and mid-length barrels, while it gives a extra space to grip the rifle, or to mount accessories.


Yankee Hill YHM-7100 upper receiver has a Spector handguard that covers the low-profile gas block.
 
No matter what type or style of handguard you finally settle on, the most important, basic factor you must account for is what length gas system is on your AR-15. Before you make any other decisions or choices, make sure you know that key piece of data.

Choose Your Use

The old architectural saying "form follows function" will really help you narrow down the type and style of handguard to get. What is the main function or purpose of your AR-15? Is it your prairie dog or coyote rifle? Are you using it to compete in 3-Gun or some other action-shooting sport? Is it your home-defense gun? Are you using it as a patrol rifle in your capacity as a law-enforcement officer? Are you shooting at long-range targets or up-close targets? Or do you just want a really cool-looking gun to impress your buddies at the range? Once you settle on the main purpose your rifle will be dedicated to, it will become easier to choose a handguard. In a future article, we'll take a look at the advantages and strengths of different types of handguards, and why certain models suit rifles dedicated to specific purposes.