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MK20 Recce Retcon: Tuning The SCAR

Since the SCAR-H was released on the market, people have been doing their ready best to transform it into a DMR. Usually, they start and end with a longer handguard and some glass — and that’s it. Today we’ll be performing the reverse: chopping down a SCAR Mk20 to fighting length.

The main issue is that the SCAR-H was never really intended to exist at all, not in the form we have it in. When FN made the SCAR-H, they basically just stretched out their smaller SCAR-L 5.56mm rifle to accommodate the larger 7.62x51mm. They even kept the skinny, whippy pencil barrel from the SCAR-L, much to the chagrin of those attempting a higher level of precision.

SCAR Mk 20 Recce Tuning

When the FN SCAR Mk20 was initially developed, 7.62 was still considered a viable long-range cartridge. By the time the Mk20 was released to the public? The swan song of 7.62 had already been sung. And though it’s not the worst for precision, it has since been surpassed by lighter, faster, wind-bucking calibers. FN FN’ing finally released the equally vaunted-and-maligned precision SCAR Mk20 in 2019 in 7.62 and followed that up with a fantastic 6.5 Creedmoor version.

We’ve seen a lot of people try and cobble together a precision(ish) rifle from a SCAR-H, but we’re trying to find that DMR-Fighting-Gun Balance from the other end of the scale. Fear not, purists — the only permanent modification took place on the barrel. FN is now shipping upgraded 1/10 twist 7.62 barrels, officially making this arguably the least desirable Mk20 barrel of them all. At least until we made it cooler. We present, the SCAR Mk20 Recce Rifle. 

INITIAL APPROACH 

As it ships from FN, the SCAR Mk20 is a bit ungainly at over 40 inches in length. Add in a silencer and you’re looking at something even longer than a Barrett 50. We had to take it down to a handier size for both use and transport. 

BARREL

The first and most noticeable alteration is the barrel. We headed over to Curtis Tactical for a quick chop-and-thread (see RECOIL Issue 54 for some of their work). Pinning-and-welding a sub-16-inch barrel isn’t something we’d normally do because muzzle devices can’t be swapped (and we love short-barreled rifles), the Mk20 SSR receiver itself limited how short we could go somewhat. 

curtis tactical turning down the Scar mk20 barrel
Curtis Tactical performed the heavy lifting of chopping down the barrel.

Ultimately it was decided the barrel would be cut short enough so that a permanently attached SureFire SOCOM SFMB-762 brake would bring the overall length to 16 inches. While the actual length is somewhere around 13.8 inches, for practical purposes we’re going to refer to it as 14 inches. 

curtis tactica scar barrel turned down

While cutting off about 6 inches does make the rifle handier, the bulk of the weight is still under the receiver. Seeing as how the Mk20 barrel is more than 4 pounds heavier than a SCAR-H’s, in hindsight some more meat should’ve been taken off while we had the barrel on a lathe. Next time. 

BUTTSTOCK

The standard stock on the Mk20 is chock full of adjustable features, but lacks the one most important when it comes to compact transport: a folding mechanism. While any SCAR stock can drop into the Mk20, we decided to keep precision in mind with the Kinetic Research Group SCAR Long Range Stock.

kenetic research SCAR stock
The Kinetic Research Group Mk20 stock has the aesthetics of OEM with added features such as the ability to fold for storage or transport.

While folding was the initial goal, there’s a lot to like about the KRG stock. It has the same toolless length-of-pull and cheekpiece adjustment, while adding a solid folding ability. It also features buttpad rotation and QD cups. When folded, it lops 9 inches right off. 

OUTFITTING

OPTICS & MOUNTS

Determining what optic(s) would be fitting for this rifle was categorically the most difficult portion. While LPVOs are practically being thrown on everything but pocket pistols these days (give it time), they still aren’t ideal here. The higher ends of magnification (such as 10x) are good enough to make shots at man-sized targets at 1,000 yards, but we’d want a little more for recce-rifle purposes such as limited-probability targets and observation. 

SCAR mk20 scope and crimson trace MRDS

What we really want doesn’t exist yet: an optic with an LPVO form factor in FFP, but with a low-magnification level of 2-2.5x and 13-16x on the high. As a matter of practicality instead of pretend, we mounted the first focal plane Vortex PST Gen II 3-15×44 scope in a Midwest Industries mount. 

Accufire thermal

For passive night vision use and up-close work, we decided on a second dot on top of our variable, in this case a Crimson Trace CTS-1250. Even with 1x variables we’re seeing this more. Not only is it easy to use when looking through tubes, any eye relief or off-angle viewing issues are eliminated. 

Never to leave well-enough alone, we decided to test out a new thermal too in the form of the Accufire Technology Incendis. 

RECEIVER & MAGAZINES

When you consider when the FN SCAR was developed, the proprietary magazines make some sense. Readily affordable SR-25 compatible magazines weren’t even a twinkle in Magpul’s eye and the quality level of other “battle rifle” magazines is always questionable. But it’s no longer 2008, and the SR-25 magazine reigns supreme for semiautomatic 7.62 use. 

There are aftermarket trigger modules available for the SCAR that accept SR-25 magazines such as Stryker and X-Products. We had an experimental one from the last days of Handl Defense on-hand so we used that, though given some QC issues seen with Handl we couldn’t recommend one if you find it sight unseen — importantly, we already know ours works. At least with Magpul magazines; others are hit and miss. 

HANDGUARD

No aftermarket here — we decided to slim down the system by removing the side rails completely. The continuous bottom rail may be replaced with a KeyMod or M-LOK version sometime in the future, or perhaps we’ll mill all the rails flush sans a few for a bipod.

TRIGGER & SMALL PARTS

FN decided that they shouldn’t try and reinvent the wheel when it came to the Mk20 trigger. They skipped making their own and rolled with the Geissele Super SCAR trigger straight from the factory. Not every SCAR trigger is bad, but the Super SCAR is always better. For a charging handle, we’ve really come to like the KDG Scarging handle when running magnified optics. The extended and angled Scarging handle makes running the gun from the prone a picnic. 

SILENCER

The original silencer for the FN SCAR literally had a hole drilled in the bottom. No, it wasn’t around very long. Shortly thereafter, SureFire was tasked with developing a specific suppressor and mount for the SCAR, and they came up with the FA762SV-BK. As with the other Mk20 articles that have graced these pages, we rolled with a SureFire SOCOM762-MINI2. Same family, more versatile silencer — we’re obviously not trying to be “clone correct” in this piece.

PAINT

While we’ve never hated FN and their amazing Technicolor shades of FDE, we wanted to try something different here. Cerakote would probably be the best option for durability, but Rust-Oleum in a rattle can is damned easy. For this rifle, we sanded and prepped and used a couple different sizes of painter’s tape to make a fractal-ish pattern and then topped with some spatter. It won’t be to everyone’s taste, but it’s quick to clean up when scratched and can be stripped in your kitchen sink if required. The exposed barrel being nearly entirely blue we didn’t notice until it was time for studio shots, and that has since been remedied with a touch of solvent. 

SCAR mk20 tuned tuning the SCAR at the range
A standard cheekweld is used for the variable, and you just pop your head up to use the 1x MRDS piggybacked on top.

All told, rattle can paint jobs may be ugly, but damned if it isn’t both economical and functional. 

BALLISTICS 

If you’re a long-range shooter, the 1/12 twist of the Mk20 may throw you off. But there’s definitely some leeway as to exactly how heavy you can go, bullet-wise. As a rule of thumb, you want to stick to some of the lighter loads. Most projectiles 168 grains and below will be fine. 175-grain+ may be hit and miss, but it largely comes down to elevation and temperature. All told, if you want to go heavier, you’ll have to try it for yourself. 

Before any sort of precision could be tested from our Recce SCAR, the system first had to be tuned for the new barrel. After that process was complete (we used a 1.7mm gas jet; see the sidebar for details on how to make your own gas jets and also tune your own rifle), it was chrono time. 168-grain Hornady ELD Match ammunition gave us an average of 2,417 fps, and after plugging data into our Applied Ballistics software, here’s the chart we came up with:

Though the projectile won’t go transonic until after 900 yards, the holds become quite precipitous beyond 400. What’s this? A 5.56mm with more work? Kind of, but with a bit more ass in the seat.  

ON THE RANGE & LOOSE ROUNDS

As expected, the Vortex Viper PST Gen II was an excellent performer and is well within the needs of our 800-yard rifle. We’re well acquainted with the MRAD EBR-7C reticle. We did opt to add a throw lever because magnification adjustments on the PST series tend to be stiff. A quick and dirty zero was performed on the MRDS by first aligning aiming points with the Vortex at 50 yards and then refining. The skyscraper height over bore is so obscene Jay Leno couldn’t get chinweld. Sometimes what works in the shop doesn’t work on the range. Next time, we’ll offset the dot. It does allow for easier ambidextrous use.

SCAR Mk 20 Recce Tuning

The Accufire Technology Incendis doesn’t have specs that’ll blow anyone’s socks off as it has a 348×288 resolution but with an MSRP below $3,000, it should certainly grab your attention. The price also attenuated any potential worries about the SCAR murdering optics. While it can be used as a somewhat clumsy handheld or as an independent optic, we found it best suited as a clip-on. Any magnification over 2.5x will fill the window. There are a lot of features we’ll be going through in a full review, but a spring morning had us catching heat signatures at 700 yards before we were limited by terrain.

Early in the range session we swapped out the SOCOM762-MINI2 for the more-quiet SOCOM762-RC2. In this case, some added length was warranted, as we were asking much from the MINI2. At the end of the day, the silencer can be popped off to fit in your rig bag anyway, so the length is only temporary. 

It was a pleasant surprise to see that shortening this barrel has appeared to tighten up the groups a bit. Five-shot groups with the 168-grain Hornady ELD Match hovered right below 0.75 MOA. 

The tuned gas and overall mass used physics to make felt recoil its bitch, and fast follow-on shots were easy. As predicted, the weight became a bear quickly when firing offhand, or we should be eating our Wheaties. This one is best from a bipod or tripod for expended use unless the world really has gone sideways.

Is this going to be the new infantry rifle? Pfft, hardly. But it does make for a rifle that’s compact, handier up-close but can still perform at longer ranges. 


FN SCAR Mk20 Recce Rifle

Overall Length: 27 inches (folded), 35 inches (extended)
Barrel Length: 14 inches
Caliber: 7.62x51mm
Capacity: 10, 20, 25
Weight: 9 pounds, 12 ounces
URL: fnamerica.com

Accessories: 
KRG Stock $499
SureFire SOCOM762-MINI2 $1,179
Vortex PST GEN II $1,999
Crimson Trace MRDS $229
Midwest Industries mount $185
Accufire Incendis $2,899


DIY GAS JETS AND SCAR TUNING

We knew we’d be breaking new ground with a barrel this short on a SCAR Mk20, and that factory gas settings, even if they worked, would be less than ideal. While aftermarket gas jets are available in many sizes at prices below $10 each — but if you don’t know where you should start then you may need an awful lot of them. 

Tuning the SCAR MK20

For reference, here are some commonly used gas jet sizes as starting points:

SCAR 16
16-inch barrel: 1.4mm
10-inch barrel: 2.0mm

SCAR 17
16-inch barrel: 1.5mm
13-inch barrel: 1.6mm

SCAR Mk20
20-inch barrel (7.62x51mm): 1.4mm


CUSTOM GAS JETS

First, let’s talk about how you make your own ports in the first place and even how you can modify them on-the-fly during tuning. Back in RECOIL Issue 38, we threaded and vented an AK gas block using vented cup-point screws; this is a similar-but-easier process. As a bonus, the vented set screws we’ll be using are made from 18-8 stainless steel. The vent size that comes standard with these screws is 1.3mm, and all we’re going to do is open up the vent a little bit when needed.

Parts required:

  • Vented cup-point set screw, size M5x0.8mm, 5mm in length (McMaster-Carr #91979A713)
  • Set of numbered drill bits
  • Cordless drill
  • Vise grips (if you want to do this during tuning)
  • 2.5mm hex key

Steps:

1) Using the drill bit size chart at the right, select the bit that’s your closest match.

SCAR gas jet

2) Grasp the vented screw with your vise grips and bore out the hole with your cordless drill. Done! If you were making these commercially, undoubtedly you’d be using a jig — just be sure not to Mongo your screws with the vise grips. 

You can make several in advance to take to the range (drug dealer baggies are perfect for storage and labeling) or do it on-the-fly. Performing this at the range can be a real pain sometimes, but you also go through far fewer vent screws. These are sold in five-packs so there’s some wiggle.

For your reference, here are each numbered wire gauge bit ODs we may use:

Drill SizeDiameter (mm)
#551.3208
#541.397
#531.5113
#521.6129
#511.7018
#501.778
#491.8542
#481.9304
#471.9939
#462.0574
#452.0828
#442.1844
#432.2606

TUNING

FN uses a special machine to determine the correct gas jet size based on rpm (from nearly two-dozen options!). Though you’ll chew through some ammunition here, if you play your cards right it won’t be too much, and no fancy machine is required. 

The first thing you need to do is determine how you want your SCAR tuned. In this article, we wanted the lower gas setting to be a truly suppressed setting. This means we want the SCAR to run normally on high gas, but only completely cycle-and-lock-back when suppressed on the lower setting. This same method can be used regardless of your goal. 

1) Remove your factory gas regulator and remove your OEM gas jet. This may require some heat in the form of a MAPP torch or a heat gun. This process is much easier with a SCAR Mk16/17 than with the Mk20, as the barrel of the Mk20 has to be pulled first to gain access to the gas jet.

SCAR Gas Jets

2) Install your desired gas jet. We’d bias on the smaller side to start, as you may end up surprised at how small you can go. These vented screws taper right into place so there shouldn’t be any concerns about penetrating into the bore unless you have some extreme gas port erosion. Replace your regulator. 

3) With the regulator in the 12 o’clock position, insert a magazine with a single round loaded into your rifle. Aim downrange and pull the trigger. Assess. 

If the round fully completed the cycle of operations and locked back on empty, you have enough gas in that position. In fact, if this is your first attempt, you may have too much gas and should try a smaller vent. 

Repeat steps one through three until you achieve a complete cycle. Double-check your suppressed setting with your desired suppressor attached. If you’re seeking out full function unsuppressed with both settings à la OEM, perform these tests with the regulator in the 2 o’clock position and then 12 o’clock becomes your “adverse” setting. 

If you’re using a three-position regulator from Mototech Engineering, we’d recommend going one size up from fully functioning to accommodate for an adverse setting.


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One response to “MK20 Recce Retcon: Tuning The SCAR”

  1. Matt says:

    The barrel is four pounds heavier than a SCAR H? Hope that’s a typo, or you’ll need more ink to hold that thing horizontal.

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  • The barrel is four pounds heavier than a SCAR H? Hope that's a typo, or you'll need more ink to hold that thing horizontal.

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