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Buildsheet: MK18 -ish Folder

The Mk18 is as iconic as it gets when it comes to SBRs.

This is not an Mk18.

This is what we think the Mk18’s punk-ass little half-brother would turn out like if its NSWC Crane papa stepped out and knocked up a young mistress.

The Mk18’s developmental roots go back to the 1960s and the original shorty AR, the Colt Commando.

NSWC Crane took inspiration from the Colt Commando when it made its CQBR drop-in upper receivers for U.S. Special Operations forces. These were 10.3-inch Colt-barreled uppers that eventually became the Mk18 when combined with surplus M16 rifle lowers from the Navy’s stocks. One of the most distinctive parts of the Mk18 is the flat dark earth RIS handguard. This was originally a Knight’s part, but Crane would later shift to a handguard made by Daniel Defense.

The Mk18 story gets confusing at this point because DD capitalized on the Mk18 name when it won the contract to produce the handguard. It wrapped its own parts in the Mk18 handguard, christened it the “Mk18 Upper Receiver Group,” and made it available on the civilian market.

The DD Mk18 upper, which is the basis of this issue’s Buildsheet rifle, is basically a DD’s recreation of the Colt-produced Mk18.

We say basically because, though the DD handguard is an authentic SOPMOD (i.e. government issued) part, the upper is an Mk18 in name only. DD used its own cold hammer forged 1:7 twist 10.3-inch barrel and upper receiver in place of the Colt parts found on the real deal.

But we weren’t trying to build a true Mk18, just a 10.3-inch AR, so we don’t have to nitpick all the differences between the DoD Mk18 and the DD version of the Mk18. We wanted a versatile, fast-handling rifle that’s easy to stow out of sight while being quick to the fight, whether serving as a truck-gun, CQB platform, or a home-defense tool.
Our shorty is set up for CQB. The heart of the Mk18ish is Law Tactical’s ARIC abbreviated bolt carrier group, giving our AR15-based carbine a fire-when-folded superpower when combined with Law’s folding stock adapter. Honestly, we got the ARIC and wanted to build a rifle around it, and this setup made a lot of sense.

We started off with our trusty lower Mega Arms billet receiver. We chose it, first, because Mega made a tightly toleranced, ambi lower, and second, because it was already Form 1’d for use as an SBR. The Geissele SDE-E trigger might be overkill for a 1 MOA gun, but it’s fast, consistent, and above all, reliable. Reptilia’s CQB grip provides an upright grip angle that’s more comfortable and efficient for rifles with short (or folded up) buttstocks. And out back, we’ve got Magpul’s Gen 2 UBR stock. It’s heavy, but it offsets the weight of the can, light, and laser we’re running up front and places the rifles point of balance right at the magwell.

We had to force the UBR to get along with the folding stock adapter using a Dremel. It took 10 seconds to radius the leading edge of the hinge-side, polymer stock cover to create clearance for the stock to fold flat against the receiver.
Up front, we added an Insight ATPIAL IR laser and a SureFire light that’s IR and white-light selectable. Both light and laser are activated with a rail-mounted, dual-button Unity TAPS switch. The nightfighter theme continues with the EOTech XPS3. Its lowest reticle settings are compatible with night vision tubes, if you want to mount a night vision monocular behind it. It’s also sitting up on a riser for a more efficient, heads-up shooting posture.

On the range

The SBR is well-mannered thanks to the recoil-absorbing, SureFire SOCOM 2 556 QD suppressor. Splits are fast and the muzzle confidently swings between targets without over-running them during transitions since the gun is well-balanced.

Accuracy is what we expect from a 10.3 running XM193 and M855, just over an inch. Though groups tightened up to just under an MOA with better ammo. Around 300 rounds in, we began experience cycling issues with the can puking carbon back into the BCG. The short, light ARIC bolt carrier didn’t have the inertia to overcome the friction provided by the combustion debris blown back into the upper receiver and slowed to the point of failing to go into battery on several shots. We lathered the BCG in CLP, and the gun was GTG till COB that day.

The gun ran unabated, whether the stock was folded or open. As sexy as the capability sounds, firing a folded-up AR should be reserved as a last-ditch, we’re-getting-ambushed sort of option.

Overall, the build is a ton of fun to shoot, both in daylight and in the night. Don a PVS-14 and use the ATPIAL to poke holes in unsuspecting things a block or more away. Chin-welded daylight (or white light) engagements with the big, bright EOTech riding high is a revelation of comfort, speed, and situational awareness when you’re used to a head-down, cheek-mashed posture on the gun.

Daniel Defense Mk18 Complete URG $1,242
Law Tactical ARIC Bolt Carrier System $369
Mega Arms GTR-3S Billet AR-15 Lower Receiver (Discontinued) $200
Magpul UBR GEN2 stock $200
Law Tactical AR Folding Stock Adapter Gen 3-M $270
Geissele Automatics – Super Dynamic Enhanced (SD-E) Trigger $240
Battle Arms Development BAD-ASS Short Throw Safety $85
Battle Arms Development Enhanced Pin Set $30
Reptilia CQG Pistol Grip $20
Radian Weapons Raptor LT Charging Handle $65
SureFire WARCOMP-556-CTN-1/2-28 $149
SureFire SOCOM556-RC2 $1,099
EOTech XPS3 $655
Kinetic Development Group SIDELOK Universal Riser $90
Dueck Defense RTS Fiber Optic Iron Sights $250
Unity Tactical TAPS $152
SureFire M300V Scout Light $429
Insight ATPIAL (LA-5B/PEQ) $1,440
Total: $6,985

Buildsheets: Past and Present

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