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The SCAR Without FN: Building a Scar at Home

SELF-ASSEMBLED FIREARMS ARE AN AMERICAN TRADITION, EVEN WHEN YOU’RE BUILDING A EURO BLASTER

As a former team guy, I was issued a SCAR MK17 when they first came out. It was light, powerful, and a bit different — but full of promise. Accurate for man-sized targets to 800 meters and beyond, it was modular, modern, and could be used in structures or wide-open terrain. 

Flat ranges and an SFAUC course or three will tell you a lot, but you never really know for sure until you deploy with it — which is why NSWC Crane safety certifies everything that touches an MK17. It’s only with hard use that you see things that the designers might’ve missed. 

Just a few short years after carrying one, and a decade ago back in 2012, I started working on SCAR enhancements, first with Handl Defense and now with Plumb Precision, and that continues to this day.

There are multiple SCAR models. The original concept, the MK16, and its commercial market variant the SCAR 16S/SCAR-L, is chambered in 5.56x45mm, and a PDW SCAR 16, the SCAR-SC, is available to law enforcement. 

SCAR-L MK16
A US Army Ranger with B Company, 2nd Battalion, 75th Ranger Regiment, fires a MK 16 SCAR-L at a close quarter marksmanship range. It is fitted with a AAC SCAR SD suppressor, SU-231A/PEQ holographic sight, flip-to-side magnifier, AN/PEQ-15 IR laser, and Insight WMX200 Visible Bright Light. U.S. Army Photo by Pfc. Rashene Mincy/Released

Fabrique Nationale most recently released the SCAR 20, a commercial market variant of the USSOCOM MK20 in both 7.62×51 and 6.5 Creedmoor. But here we’re focusing on the most popular, the FN SCAR 17S (“SCAR-Heavy”) and its MK17 USSOCOM variant. 

The MK17 operates closer to the edge of its design limits; it’s not an AR or an AK — it’s a big-block 460 stuffed into a ’69 Mustang. The capability and promise of the platform can be compromised without careful consideration of your modifications. 

Most know that SCARs can be rough on optics, and some suppressors can give them a bumpy ride. If you start overdriving the gun, you might just break it. 

The SCAR aftermarket has a small-but-robust number of companies addressing the concerns of commercial market SCAR users. 

Mk-17
MK17 SCAR

Triggers, trigger modules, barrels, bolts, handguards, stocks, grips — nearly all of the small parts for a SCAR are available from sources other than FN, though official FN parts can be used when available. 

Therefore, we find ourselves on the verge of something that’s a hallmark for any weapon system — could a SCAR be made at home, from purely aftermarket parts? 

The answer is a tentative yes. It’s not simple or easy right now, but it’s getting a lot easier. Even better, you’ll be able to select from an unprecedented number of calibers for your build. 

Note that some of the parts shown in this build are my own from Plumb Precision, but none are hard requirements for rolling your own. 

STEP 1 — GET YOUR RARE COMPONENTS TOGETHER

At the time of this writing, the bolt carrier, internal rails, and receiver back plate are the hardest to find. Midwest Gun Works has them all occasionally but not regularly. 

The aftermarket is addressing each of them, but until you have these three items, do not pass “go.” 

Bolt Carriers: With non-reciprocating carriers, first seen on their CSR 20 SCAR rifle, now shipping from FN, new and used versions will become more plentiful. 

SCAR BCG from Midwest Gun Works
SCAR BCG from Midwest Gun Works

While those are FN parts, a universal bolt carrier group should be available this year. MK17 OEM bolt carriers can also be lightened to work with other calibers. As a point of reference, the 16S BCG weighs about 550 grams, the 17S 600 grams, and the MK17 636 grams. 

Internal Rails: This rail guides the bolt cam pin in the receiver, locking and unlocking the bolt. Imperial makes these, and more options will be available, but you can make them yourself if you have a small mill. Midwest Gun Works is still currently your most likely option to snag one unless you DIY. 

Back Plate: This is the key component. These aren’t simple to make, so the straightforward way is to get the OEM version from Midwest Gun Works. Hopefully, by the time you read this, an aftermarket version will be coming online from Imperial Arms. 

Suppressed SCARs combined with metal backplates can exacerbate issues of canted or broken screws. 

STEP 2 — SELECT AN 80-PERCENT OR COMPLETE UPPER RECEIVER

Imperial Arms has been making upper receivers that are compatible with all the parts listed and SCAR OEM parts and more recently rolled out 80-percent upper receivers as well. You build them yourself, and they come ready for your router with a jig. 

If you want to pursue the 80-percent route before the BATFE does its thing, or if you want the finished item shipped to your local FFL, Imperial Arms has it sorted.

No FN SCAR Build (2)
Imperial Arms has an 80-percent receiver kit. It provides the jigs and fixtures. There are some tools required to complete the receiver — it’s not difficult, just different.

The Imperial Arms 80-percent kit comes with all the brackets and jigs you’ll need. It’s not much more complicated than making an AR receiver, but it sure is different. 

Detailed instructions for the process are best gleaned from Imperial Arms’ YouTube page. 

Fasteners: Both Shooting Sight and Mototech Engineering provide receiver screw kits. You’ll need the following fasteners to complete the upper receiver:

  • (1) M5x10
  • (9) M4x6 
  • (4) M4x8
  • (4) M4x12
  • (2) M5x12 (with Torx T25 head)

A bit of blue or green thread locker is recommended on all fasteners sans the barrel screws (see “The Glue That Binds Us” for more details). 

No FN SCAR Build (3)
Imperial Arms has an 80-percent receiver kit.

Speaking of barrel screws, Shooting Sight produces small brass shims if you want to use regular flat screws instead of beveled screws. As helpful as the FN barrel quick change assemblies are, those fasteners aren’t an easy find. 

A small company called Rebel Defense makes not only the quick-change barrel screw housing but also a metal brass deflector.

STEP 3 — SELECT CALIBER, TRIGGER, AND LOWER 

Barrels: One promise of the FN SCAR is a barrel that can be swapped easily without too much fuss. That’s largely true, with the caveat that you need another barrel. 

FN replacement barrels, like most OEM SCAR parts, are expensive and hard to come by. Additionally, factory MK17 barrels are 1:12 twist, optimized for NATO M80 ball and USSOCOM derived MK 319 SOST ammo. 

Not only do people want more twist rate options, but they also want more calibers.

Thus, Deadshot Barrels has been delivering barrels for the MK16, MK17, and 20S in various calibers and twist rates for years. You want a 20-inch 6.5 Creedmoor barrel or a 13-inch .338 Federal for an MK17? Deadshot Barrels has you covered. They also have various fluting options, conversion services, and even bolts.

No FN SCAR Build (5)
The Deadshot Barrels gas block has extra vent holes at the bottom of the gas piston stroke. It allows for additional pressure relief and helps prevent overdriving the gun.

There are three barrels used in this build: 10.5-inch 5.56mm, 13.5-inch 7.62×39, and 16-inch 7.62×51. 

The 5.56mm and x39 barrels are from Deadshot, while the 7.62×51 is OEM. Deadshot barrels have a thicker profile and several options for fluting. Their gas block, in particular, deserves some attention. 

Inspect a factory gas block and you’ll notice a vent at the bottom that allows excess gas to bleed off. Deadshot Barrels has refined and improved the concept (See “Superleggera SCAR” in RECOIL Issue 43).

Triggers: Many contend the greatest improvement to the SCAR comes from replacing the trigger unit. The early generations of MK17 triggers were less than spectacular. Some would feel more at home in an M249 or M240 than a carbine, though they’ve improved substantially over the years. 

Geissele and Timney brought solutions to market, with the former showing up on some issued rifles. Having shot them all many times, they all improve feel. 

No FN SCAR Build (10)
The SCAR trigger module requires a lot of proprietary parts that might be hard to find (above).

A later addition, and my preference, is the Shooting Sight SCAR Trigger. It’s a two-stage like the Geissele Super SCAR, but with some subtle differences. 

Milled from rolled tool steel and then polished, it provides a super sharp break. The Shooting Sight is also fully adjustable and can be dialed into your liking, between 4.5 and 5 pounds of pull. Finally, it reduces hammer swing by about 3 milliseconds.

Lower: The lower receiver of the FN SCAR isn’t a receiver at all but instead a trigger module. It’s not a serialized part like on an AR-15, but it closely resembles an AR lower. 

One initial complaint about the MK17 was the proprietary magazines. Unlike most factory SCAR parts, the OEM trigger module can be found cheap and will work just fine with SCAR magazines. 

An early project I was known for was SR-25 mag compatible trigger modules, finalized back in 2014. You can currently get an SR-25 lower from X-Products, Stryker Enterprises, and Imperial Arms.

No FN SCAR Build (9)
Or you can go with the Imperial Arms MTM trigger module that uses AR parts

The Cypher MTM (Mil-spec Trigger Module) trigger module from Imperial Arms is a game-changer for the cost of homebuilding an MK17. 

It’s lightweight at 9 ounces and cuts the cost of your build by about half, through the use of commonly available AR-10 triggers, grips, and other lower parts. Since the MTM uses AR parts and not FN parts, they’re far more affordable to build from scratch than other lowers.

One of the trigger modules seen here was developed for the then-unfilled AK SCAR variant requirement of the SOCOM program. In the final stages of commercial development, the use of the AK trigger module allows rock-n-lock mags in 7.62×39, 5.45×39, and 5.56mm. 

Between the Imperial Arms MTM and this lower, you can run a very broad spectrum of calibers. 

STEP 4 — TUNE GAS TO CARTRIDGE AND BOLT CARRIER GROUP

This is where an absolutely critical component comes into play: the gas regulator. OEM bolt carriers are different weights, and all of the calibers have different powder charges. 

Then, once you throw silencers into the mix, the FN two-position selector just doesn’t cut it. Tuning the gas system to work with your specific load is critical. 

No FN SCAR Build (6)
The Deadshot Barrels gas block has extra vent holes at the bottom of the gas piston stroke. It allows for additional pressure relief and helps prevent overdriving the gun.

You can purchase custom gas jet inserts for barrels, but the Mototech Engineering Gen3 SCARburator makes the process painless and toolless. It drops right into an FN or Deadshot Barrels gas block. 

The Gen 3 SCARburator provides 10 gas settings via a dial function that allows for progressive gas release into the system. 

Deadshot Barrels provides an appropriately sized gas jet and the overflow vent for whatever barrel you purchase. This combination can resolve issues with suppressors that plague the system. 

Tune the gas to the lowest setting, then increase the gas flow to a point where the bolt will lock back on the last round in the magazine. 

After that, simply click the regulator to one more setting higher to allow for variance in ammunition. 

STEP 5 — SELECT STOCK, HANDGUARD, AND ACCESSORIES 

The SCAR stock has been given the “UGG boot” moniker for its low fashion but high utility, and also because it looks like a boot. Even though it’ll eat your beard, it’s quite effective. 

Outside of FN there are several drop-in stock assemblies for the SCAR, the most popular being an ACR stock conversion from Kinetic Development Group (KDG) though recently discontinued. HAGA Defense is soon to release a modified Magpul Zuhkov stock, Vltor has an AR stock adapter, and Kinetic Research Group (KRG) has developed a fully adjustable folding precision stock (see “MK20 Recce Retcon” in RECOIL Issue 55 ). 

While you should use caution when using alloy back plates on SCARs, Picatinny versions are available from JMAC Customs. Fine-tuning your gas system and other advances in accessories may very well mitigate or eliminate issues with them. 

No FN SCAR Build (1)

Imperial Arms makes an injection-molded, glass-filled polymer back plate adapter that allows the fitment of HK UMP and B+T stocks as well as SB Tactical SBT Braces. 

It has tight fitment with no rattle in the stowed or deployed positions, and since it’s polymer it still provides some required flex during cycling. 

Otto Arms has been making collapsing PDW stock assemblies for the SCAR for years at this point. If an MK16 SBR is your intent, you need to look at their stock. It gives you the ability to make the gun really small, yet still functional. Even though it’s a PDW stock, it has long cheekpiece, which provides good alignment for optics. 

There are also a multitude of handguard options. KDG’s MREX is the most prolific of the SCAR handguards. 

The carbon-fiber handguard I developed at Handl evacuates heat and isolates sensitive components from receiver recoil forces. It’s used on the 7.62×39 variant pictured in this article. Midwest Industries produces a well-regarded SCAR handguard. 

For handguard accessories, Parker Mountain Machine produces rail extensions that are slim and in line with the receiver profile. 

No FN SCAR Build (4)
The Mototech Engineering SCAR Charging Handle has great capture and feel due to its aggressive knurling, which also makes it very effective in inclement weather.

They also make the REP-LOK which retrofits the OEM lower rail assembly to M-LOK. One of these is featured on the 5.56mm barrel assembly in this article.

We all know about the reciprocating charging handle, and it should be noted that current aftermarket charging handles won’t work with the new SCAR non-reciprocating bolt carriers. KDG, GG&G, TangoDown, and others offer charging handles. 

The KDG SCARging handle is a favorite and has a nice downward-sweeping angle to keep your knuckles away from optics mounts. Mototech Engineering also developed their own charging handle that’s very well designed with knurling that provides good purchase. 

With wet gloved hands (common in the Northwest), the Mototech charging handle was always easy to use and is a standout. 

PARTS LIST & COST

  • Imperial Arms Cypher X 17 receiver $445
  • Imperial Arms MTM receiver $475 (AR parts not included)
  • Imperial Arms back Plate Adapter $90
  • Plumb Precision MK17 backplate $120 (TBD)
  • Plumb Precision MK17-AK receiver $510 (TBD)
  • Plumb Precision MK17 Universal Bolt Carrier Group $806 (TBD)
  • Shooting Sight Trigger $275
  • Shooting Sight barrel extension screws $7.50/ea
  • Mototech Engineering Gen3 SCARburator $150
  • Mototech Engineering Back Plate TORX screw set $20
  • Mototech Engineering Cam Pin CrN coated $40
  • Mototech Engineering Receiver Screw TORX screw set $10
  • Mototech Engineering Locking Plate $9
  • Mototech Engineering UBER chagrining Handle $90
  • Mototech Engineering Firing Pin Retainer $9
  • Deadshot Barrels 7.62×39 barrel assembly with bolt $1,250
  • Otto Arms PDW3 Stock $450
  • Firing Pins – Stainless SCAR firing pin $28
  • Rebel Defense Barrel Screw Housing $75
  • Rebel Defense Aluminum Brass Deflector $65

LOOSE ROUNDS

Yes, you can build a SCAR-type firearm at home. It requires patience, persistence, and a lot more hands-on effort than an AR. There isn’t a one-stop-shop outside of FN, so prepare yourself for some scrounging and searching. 

Still, it doesn’t require a shop press, special tools, and jigs like an AK. 

The good news is if you play your cards right, once you’re done you’ll be able to run all the calibers of both of those other platforms in a single gun. 

The ability to build your own SCAR-type firearm, outside of FN, will only continue to get better. Happy hunting. 

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