How to Install a M203 Grenade Launcher

Tom schools us on how to install an M203 Grenade Launcher on your AR15 or M16 rifle.

Here's the Lewis Machine & Tool (LMT) M203 37mm launcher installed on the Brownells BRN-601 AR-15 rifle.

USA –-( We’re giving away a complete retro classic AR-15 and grenade launcher package, and you’re probably going to be the lucky winner, but just in case you’re not, you can add your very own M203 Grenade Launcher to a rifle you already have. Here, we’ll show you mounting options and exactly how to do it.

The installation process shown here is for a standard M16A2 20-inch rifle barrel with a .625” outside diameter. If that’s not your rifle, no worries, there are additional mounting options for M4 carbine barrels using a GI-type barrel nut at the rear. You buy the LMT M203 unit separately and then choose the right mounting kit option from the following choices.

M203 20-inch Mounting Kit with Leaf Sight

The “standard” original mount replaces the triangular handguard with black polymer versions containing internal aluminum heat shields. This model installs completely behind the A2-style gas block and front sight housing assembly and requires a rifle with the standard mil-spec barrel nut. The replacement handguard comes with the leaf sight. We’ll walk through the installation steps in detail in a minute.

M203 Carbine-style Mounting Kit with Leaf Sight

If you have a more recent M4-type rifle that uses or has a circular front cap and the .625″ outer diameter cutout just ahead of the gas block, then carbine-style mounting kit is what you need. Like the 20-inch barrel mounting kit, this one also comes with a leaf sight and replaces the whole handguard assembly.

Here's the carbine-length mounting kit. Note the replacement handguard and leaf sight. Here's the carbine-length mounting kit. Note the replacement handguard and leaf sight. M203 Quick-Detach Mounting Bracket

This component is not a complete mounting kit but rather an add on to either of the rifle or carbine mounting kits that allows quick attach and detach of the launcher.

M203 Grenade Launcher Installation

I found that installation of the M203 launcher on the Brownells BRN-601 was straightforward once I found the instructions. While a simple process, it did require a bit of muscle and brute force at times, so an extra pair of hands might help.

Step 1 is to remove the existing handguard by pulling down the delta ring to free the two halves.

Since the launcher replaces the handguard, the first step (after making sure the rifle is unloaded!) is to remove the standard triangular handguard from the BRN-601 AR-15 rifle. Technically this doesn’t require tools, but a handguard removal tool will really help as the spring under the delta ring is under some pressure. I didn’t have one and relied on muscle power, so it’s doable either way. Just pull the delta ring towards the receiver enough so that you can rotate the base of the handguard out from underneath. Once you do that, the handguard simple pulls out of the triangular support behind the A2 gas block and sight housing.

LMT M203 Grenade Launcher Installation Parts Check

Now that the barrel is exposed, you can slide the front mounting bracket into place. There’s a polymer collar that protects and insulates the barrel. It’s a bit tricky to seat the two halves of the spacer inside of the metal bracket when you get to attaching the M203 to the bracket, but persevere, the end result is worth it.

LMT M203 Launcher Installation U Clamp

This “U” clamp provides the hard mount for the M203 Grenade Launcher at the forward end. At the rear, the launcher slides into the barrel nut.

LMT M203 Launcher Installation U Clamp

The trigger guard on the M203 Grenade Launcher swivels around a forward mounting point but has no rear anchor. For that, the unit relies on the lip of the magazine well. The steel guard has a bit of flex, and you have to compress it slightly so that a notched area compresses against the lip of the magazine well. That's what keeps the trigger guard closed. You’ll need to make sure the trigger guard is in place before completing the next step of bolting the M203 to that forward “U” clamp around the barrel.

LMT M203 Launcher Installation Trigger Guard Tips

As you insert the rear of the M203 Grenade Launcher into the barrel nut area, while taking care to make sure the trigger guard us positioned against the mag well properly, you’ll rotate the forward end of the M203 to mate with the barrel “U” clamp.

The rear of the M203 has a ridge that fits into the barrel nut area.

The M203 has a hard ledge that butts against the clamp, so you might have to apply some muscle to get everything seated. I had to bend the trigger guard ever-so-slightly to make everything fit properly, so be prepared for that.

The rear of the M203 has a ridge that fits into the barrel nut area.

The mounting kit comes with a section of steel wire. That’s the double-secret-mount insurance. The bolt that attaches the M203 to the forward “U” clamp has holes in the head and nut on the opposite side. Run the wire through the bolt head about half way and then start twisting the two wire ends together tightly against the bolt head. Continue this twisting until just enough of the wire is a two-strand “rope” to reach around the front of the M203 Grenade Launcher to the nut on the opposite side. Run one end of the wire through that nut and resume twisting on the far side for maybe another inch. Clip the excess and tuck it away around the back of the mount, so it’s out of the way. It sounds complicated so refer to the pictures here. All of this provides a bit of security against the bolt loosening and the forward section of the mount disconnecting.

LMT M203 Launcher Installation Tips

At this point, you have a fully mounted launcher, but with no handguards. The replacement black handguards with aluminum heat shields install just like the originals you used in the first step. Insert the forward end of one handguard half into the front triangular mount while pulling the delta ring towards the receiver until you can seat the rear ledge of the handguard under the delta ring. Repeat for the other half. Again, you can do this by hand, but a handguard removal tool makes it a lot easier.

LMT M203 Launcher Installation LMT M203 Launcher Installation

Last, install the leaf sight. This just screws into the top of the handguard. You can perform this step first if you like, I just found it easier to complete all the other steps without the leaf sight in the way. If you have to muscle any of the other parts, you don’t want to damage the leaf sight in the process.

LMT M203 Launcher Leaf Sight

The installation process for a carbine is virtually identical as the most significant differences are the length of the handguard and the circular front handguard mount. If you want to make an M203 a semi-permanent addition to your rifle, just use one of the standard mounting kits depending on whether you have the 20-inch rifle or a carbine. If you want quick-detach capability, be sure to order the M203 Quick-Detach Mounting Bracket.

The handguard leaf sight works of the M203 Grenade Launcher with the existing A2 front sight to line up your target at ranges from 50 to 250 meters.
Tom McHale Tom McHale

About Tom McHale

Tom McHale is the author of the Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon. You can also find him on Facebook, Twitter, Instagram, and Pinterest.

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