How to Pick the Best AR Handguard Length

Short Story: Selecting a handguard length is more than just an aesthetic choice; there is a significant element of safety involved. Too short, you could burn your fingers; too long, you could blow them off.

When you select a handguard length, you must consider two main things: gas system length and barrel length.

Gas System Length vs. Handguard Length

The measurement associated with the nominal gas system length will determine the minimum safe handguard length for a firearm. Principally, you should avoid a handguard that will expose the gas block or gas tube (especially for pistols and carbines). The gas system of your AR gets very hot. If your handguard does not cover the gas tube and gas block completely, you risk having your hand come in contact with 700°F metal. The image below illustrates what you DON’T want:

How to Determine Actual Handguard Length

Nominal handguard length is generally stated as the actual handguard length. When installed, this generally translates into the distance between the forward face of the upper receiver (i.e. excluding the barrel nut threads or mounting lug), and the muzzle end of the handguard. There may be some consideration for the jam nut, if applicable to the mounting system. Check your manufacturer’s specifications for actual measurements.

The specific guidance in this article focuses on the Aero Precision Enhanced M-Lok handguards, as we use these, exclusively, on our builds.

How to Determine Minimum Handguard Length

Your nominal gas system length (i.e. pistol length, carbine length, mid length, rifle length) is associated with the length of the gas tube and the distance from the front edge of the barrel extension to the gas port on your barrel (A)

Because the barrel sits into the upper receiver, the practical measurement for calculating handguard length excludes the portion of the barrel seated in the receiver. A more practical measurement is from the barrel extension flange to the gas port. However, we need to express the gas system measurement in terms that translate to meaning for the handguard length. As such, we provide the distance from the front face of the top rail on the upper receiver (i.e. excluding barrel threads/mounting lug) to the gas port (B).

We also must factor in the length of the gas block. The gas block site over the gas port and extends both behind and in front of the gas port. The typical gas block is 1″ long, with about 3/4″ sitting forward of the center of the gas port. As such, we must add this additional value to the minimum handguard length (C)

The table below illustrates the standard measurements for the nominal gas system lengths. Note that measurement C (in bold) represents the recommended minimum handguard length for a given gas system length.

The table below shows general compatibility of Aero Precision Enhanced M-Lok handguards with nominal gas system lengths (note that this does not reflect the other factor, barrel length, which will be covered in the next section).

* Gas block will peek out, but this is generally not an issue for these rifles, because you will probably be shooting supported (e.g. on a bipod). Just be cautious of the gas block when handling the rifle after any substantial strings of fire.

Barrel Length vs. Handguard Length

The length of your barrel will inform the maximum safe handguard length for a firearm. While there is consideration to the muzzle device/attachment, you generally don’t want the end of the handguard to be forward of the muzzle of the barrel (excluding the muzzle device). The reason is simple, you won’t want to have any risk of dangling one of your fingers in front of the muzzle.

How to Determine Maximum Handguard Length

The first consideration when calculating the appropriate handguard length for a given barrel length is the type of muzzle device that will be installed, if any.

Note that the closer the end of the handguard is to the muzzle, the more likely you will get burned or otherwise injured. Push the envelope at your own risk.

If you are either not installing a muzzle device or are installing a standard “side blast” flash hider, compensator, or muzzle brake, your handguard MUST NOT extend beyond the crown of the barrel. Said another way, the crown of your barrel should extend beyond the handguard. It is typically ok for the threads to lie within the handguard for most muzzle devices.

Recessed Muzzle Devices

DO NOT use a muzzle brake, compensator, or flash hider that directs the hot gas radially (up, down, to the sides) with a handguard that extends beyond the crown of the barrel. You will, at the very least, get some nasty stuff happening to your handguard where the gas is expelled. At worst, you will burn the sh*t out of your hand or lose fingers. The image below illustrates the risk:

With that said, the barrel may be shorter than the handguard IF either:

1. You are using a blast forwarding device (e.g. Noveske KX3). In this case, the handguard must not extend beyond the forward face of the device.
2. You will be using the firearm with a dedicated suppressor (i.e. you will not fire the gun without the suppressor installed). In this case, the handguard must not extend beyond the forward face of the suppressor.

The minimum handguard length, in this case, can be calculated by considering the composite length of the barrel and muzzle device/suppressor, with consideration of the overlap between the two.

Example Calculation of Correct Handguard Length

For demonstration, we will assume that we have selected a 10.5″ barrel and a 2.5″ blast-forwarding device (BFD).

The first component of the calculation is the combined length of the barrel and muzzle device. When using a BFD or suppressor, you can use the combined length of the device and barrel barrel when calculating the safe handguard length. Using our example, the combined length of the barrel and BFD would be 13″.

However, this isn’t entirely complete or accurate.

Next, we have to adjust the combined length to account for the overlap between the two. To calculate the overlap, we must account for the length of the threaded portion of the barrel, less any crush washer or shim depth. The standard threads on an AR-15 barrel are between 0.600″ and 0.625″ and a crush washer (uncompressed; if applicable) is 0.15″ in depth. So, the overlap is calculated as:

When we combine these two components, we get the theoretical maximum handguard length. The calculation is as follows:

The table below minimum barrel lengths for the different lengths of Aero Precision Enhanced M-Lok handguards.

1: Handguard shall not extend beyond the crown of the barrel.

2: Assuming a 2.5″ blast forwarding device or dedicated suppressor (for context, the Noveske KX3 “Flaming Pig” is 3.25″ long). Use Calculation 3 to calculate the appropriate length for your barrel and muzzle device/suppressor.

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