Folks often ask us how to select a handguard length for a given gas system, or they ask what barrel length is needed to achieve a desired positioning of the muzzle device relative to the handguard. This post aims to help explain the factors involved and provide an easy guide to designing your dream setup.
Below you'll see an ordinary AR-15 Upper Receiver with a barrel and low-profile gas block installed:
The distance 'A' is from the Upper Receiver Front Face to the Gas Block Shoulder, and depends on the barrel's Gas System Length.
|Gas System||Distance 'A' from Receiver to Gas Block Shoulder|
Distance 'B' is from the Upper Receiver Front Face to the Muzzle and is always the barrel length minus1⁄8". So if your barrel length is 141⁄2" then the muzzle will be around 143⁄8" from the Upper Receiver Front Face. This assumes you're running a typical gas-operated AR-15 system, not a blowback system like 9mm.
Example 1 (Minimum Handguard Length to Cover a Low-Profile Gas Block):Let's say you have a 141⁄2" barrel with a carbine-length gas system. According to our handy table above, the gas block shoulder will be about 7 5⁄16" away from the receiver. Most low-profile gas blocks are about 1" long, so assuming you want the handguard to completely cover the gas block, then your handguard needs to be at least 1" + 7 5⁄16" = 8 5⁄16" long. The nearest commonly-available handguard size is 9", so you need to shop for a 9" or longer handguard.
Example 2 (Recess a Muzzle Device Inside the Handguard): Suppose you want to use an S&J Nano Linear Compensator and you want it to be recessed inside the handguard so that the tip is flush to the handguard end. Looking at the compensator product specs we learn that the compensator is 1.5" long. Suppose your barrel is 12.5" long. Add those lengths and you get 14", but you need to subtract 1⁄2" for the overlap when you screw the compensator down onto the muzzle threads. Fourteen inches minus 1⁄2" leaves 131⁄2". So we go shopping for a 131⁄2" handguard, (a relatively common size), and when installed we should find that the handguard end will closely (give or take an 1⁄8" or so) line up with the tip of the muzzle device.
Example 3 (Fake Suppressor, Reverse-Style): Now let's say you're building a rifle and you want to have a Fake Suppressor, Reverse Style, a Midlength gas system, and a Front Sight Gas Block. How long does the barrel need to be to accommodate the FSB and the fake suppressor? Looking at the table above, for a Midlength gas system the gas shoulder will be at 9 5⁄16" from the receiver. Our Front Sight Block is about 21⁄4" long including the bayonet lug. Add those together and we get 119⁄16". Looking at the Fake Suppressor, Reverse Style product page, it says we need 125mm of clearance between the gas block and the muzzle thread shoulder. Convert that to freedom units we get 4.92". Generally a barrel threads are about 5/8" long so we add it all together: 119⁄16" for the barrel and gas block, 4.92" for the fake can, plus 5/8" for barrel threads, we get 17.1". So we're going to need at least a 17.1" barrel, and if you're buying off the shelf that means likely an 18" or 18.6" barrel and that will leave about 1" to 11⁄2" of space between the Front Sight Block and the fake suppressor. And the reverse suppressor will give the appearance that you've got a 13 inch barrel with a real suppressor on it.
Example 4 (Exposed Gas Block): Now suppose you want a free-float handguard and an exposed gas block with picatinny rails so that you can mount a bipod to the bottom of the gas block. Let's assume a 16" barrel with carbine-length gas system. Looking at the table above, your gas block rear end (closest to the shooter) will be at 75⁄16" away from the Upper Receiver. So you need to choose a 7" free-float handguard, and you'll have about 5⁄16" of clearance between the handguard and the gas block.
Now what if you have an A1-style birdcage with an open bottom -- will the muzzle blast dirty the bipod when it's folded? Assume your bipod is 7" long when folded, add it to 7 5⁄16" and you get about 14.3", a healthy 11⁄2" behind the muzzle.