Lengths guide for AR-15 handguard, muzzle devices relative to barrel

We get a lot of questions like "How long does the handguard need to be to cover my gas block?" or "Do I have enough room on my barrel for this barrel shroud/fake can?" and "Is a sling just a boneless carry handle?" So we made up some drawings and formulas that will hopefully answer most of these questions forever and then we can make ourselves obsolete and resort to selling organs.


(Click image if you want a higher resolution image)

Question the first: how long of a handguard do you need for a rifle-length gas system? Let's assume your gas block is about 1" long which is the case for most gas blocks. If you want to cover the gas block you'll need a handguard at least 12.7+1 = 13.7" long. In this case you could go with a 13.5" and have the the gas block slightly outside the handguard, or go up to 15" to cover it completely with room to spare. (Obvies, we're measuring from where the receiver ends and the handguard starts.)

Another example, supposing you have a carbine-length gas system, the drawing shows the gas block shoulder at 7.3" from the receiver, so we add our 1" for the gas block and that puts you at 8.3". Bingo bango you'll need a 9" or longer handguard to cover that hot diggity block.

If you have a longer gas block which sometimes people use if they want to cover up the carnage left behind after removing a Front Sight Block, then you'll have to account for that extra length.

Now after the handguard is chosen, let's say you want to install a barrel shroud like one of our Reverse-style fake suppressors, then you need to know if you have enough free space on the barrel. Using the same drawing we can find the amount of length available between the gas block and muzzle.

For this question we choose to measure off the thread shoulder. Here's a red arrow™ to make it clear what I mean by the thread shoulder:


(Click image if you want absolutely nothing to happen)

The Reverse Fake Can product page says it needs 125mm of free space behind the thread shoulder. Converting to Freedom Units that's roughly 4.92 inches. Supposing you've got a 20" barrel with a rifle-length gas system, the drawing shows there's 5.6" available behind the thread shoulder, so we are good there.
Now what if you've got an upper that's not shown on the drawing, you've got a 16" barrel with a carbine gas system, aka the classic M4 profile with a grenade launcher notch. Will it fit? In the bottom right corner of the drawing is a formula chart. for Carbine gas system you do [barrel length' minus 9 to find out how much space you've got. 16" minus 9 is 7", so lots of room for that fake can.

Now if you had a MRA 18.6" barrel with a rifle gas system, then we look at the chart and run the Rifle formula, 18.6 minus 14.4 equals: I don't know but it's deffo less than 5. So that reverse fake can won't fit there, even if your barrel pops a blue one and musters an extra half-inch.

These formulas should cover every situation for free-float 1" gas blocks.

Ok, you say, but what about my 14.5" carbine upper with A-frame/front sight gas block? We gotchu covered too, see the drawing below:


Same spit, different location.

You can see less room for barrel shrouds/reverse fake cans in these situations. The 4.9" fake can won't fit anywhere. Maybe time to upgrade your gas block to low-profile, OR check out the limited-er selection of shorter fake cans that don't take up as much space, for example we've got this XM177-style barrel shroud that takes up 2.83" behind the thread shoulder. Call it 2.9. Either way still lots of room in all barrel examples here. On a 12.5" carbine you might be in trouble --- formula says 12.5 - 10.1 = 2 and a bit, so nagunnawork.

Anyway we hope this was educational and uninspiring.
If anything is not 100% clear please post questions below and we'll do our best to confuse you further.

If you like printed versions here is a link to a PDF of the drawings that you can print enlarged for over the mantle. Comes with bonus gas tube/gas system 'gauges' that really work if you print on 11x17 at 100% scale.

Great stuff TNA... just downloaded for future reference. Will be starting my next build soon.

Nice work TNA

Very nice, always good to have such info handy.

Very nice info, I’ve been looking into this recently and this answered all my questions.

that is pretty darn nice of you guys.
wondering if you have something similar for the few guys that are trying to build a 308 platform.
asking for a friend

tagged. Thanks.

Unreal guys thanks for this

Newbie question here... since ARs are restricted just like pistols, are 7.5inch barrels legal in Canada?

Originally Posted by mabuhay

Newbie question here... since ARs are restricted just like pistols, are 7.5inch barrels legal in Canada?

Yes