The AR-15 family of weapons is no stranger to controversy. From the early days of initial fielding during Vietnam to modern-day Afghanistan, there have been detractors highlighting faults (real or perceived) about the AR.

With more than a half-century of real-world use, the AR is the most mature rifle in human history. Seemingly every year, a new caliber or configuration hits the market. There’s an endless chain of products for every single part of the system, down to individual springs and pins. Barrel lengths run the gamut from a tiny 4.5 inches all the way to 28-inch monsters. You can nab a black powder muzzle-loader AR, and even attach uppers that are crossbows and soda launchers; at the other end of the spectrum are belt-feds. After the M-16, Eugene Stoner tried his hand at making a universal, modular rifle with the Stoner series — not fully realizing he already achieved his goal.

But that doesn’t mean there aren’t real, tangible downsides. As the role of the AR expanded, so did what we can’t necessarily call “design flaws,” since they’re well outside the original intent of the rifle, but “engineering challenges” instead. Once you transform the system from a 20-inch rifle to the myriad of applications we mentioned, Ole’ Murphy rears his ugly mug. A bolt lug breaks off under pressure it wasn’t designed for, extractors need to be beefed up, recoil systems require further refinement, and on and on.

In 2010, it looked like the M4 might finally be replaced as the mass-issue service weapon for the United States military when they announced the Individual Carbine trials. While that’s an article in and of itself, the end result was that while many rifles performed better than an M4A1, none by enough of a margin to justify wholesale replacement.

Exactly when will the AR be fully replaced? The first step will be coming up with new ammunition, and then a system will be built around it. Eric Graves of Soldier Systems contends we’ll have M4s in U.S. military inventory until at least 2040; bear in mind that some National Guard and reserve units were still fielding M1918 BARs until the 1970s.

The rifles you see in the following pages weren’t all direct contenders in the Individual Carbine trials, but they all at least attempted to address some downsides of the AR by making them more inherently modular rather than further tweaking a vintage design. This list is by no means complete, as every year a new “SCAR killer” emerges. Some, like the Bushmaster/Remington ACR, are no longer made, while others, like the FN SCAR itself, continue to chug along despite their detractors. All on this list made a mark along the way and share features that we anticipate seeing on whatever eventually supplants the M4 carbine.

FN SCAR

Overview

While HK started the modern shove-a-piston-into-an-M4 craze, FN kicked off the great über-carbine race. They began designing the SCAR system of weapons way back in 2004, and SOCOM officially began fielding them in 2009. Later that year, FN began selling them in semiautomatic form to civilians. Of course, and as usual, early adopters paid for the pleasure.

Regardless of how you personally feel about the SCAR, every next-gen carbine that came out after was hailed as a vaunted “SCAR killer.” But as of yet, the only true SCAR killer is the M4A1.

Design

While the AR-15 and variants have been heavily modified over the years, FN decided to start from scratch and address shortcomings up front. SCAR bolt lugs are much stronger than standard AR lugs, with one fewer due to their beefy nature. The gun comes with an adjustable gas system, so no fussing with aftermarket gas adjustment when running suppressed. The fully ambidextrous design was accomplished without kludgy add-on parts and includes the now-more-common short-throw selector. The barrel has a swappable gas jet, negating gas port erosion.

It’s also a piston gun, which works great in a rifle designed for one in the first place. Because the recoil spring is contained within the upper receiver, the Ugg-boot OEM stock can be folded for transport.

FN made the SCAR in both 5.56mm and 7.62x51mm (the latter dubbed the SCAR-H), but later came up with a common receiver conversion for the SCAR-H when 5.56 models fell out of military use in 2010. Other models were developed over the years, such as the SCAR PDW (hey, FN — can we get one, pretty please?), the FNAC, and the precision-oriented and much larger SCAR Mk20.

What We Like

The SCAR’s handling and controls undoubtedly inspired all that came after it, especially the integral ambidextrous controls and the short-throw selector. But that alone doesn’t account for the longevity of the rifle. Of every rifle listed here, more companies make aftermarket parts for the SCAR than any other, by a fair margin. The SCAR barrel can be swapped without a vise or many special tools, though doing so in the field (as many have written about) is a LARPer pipe dream; it’s meant for unit armorers.

What We Don’t

While a reciprocating charging handle was originally a SOCOM requirement, that’s no longer the case. The FNAC and PDW can swap between reciprocating and not, but neither those nor OEM conversions are currently available. As such, many a thumb has become a victim. And it’s not just the reciprocation, but the length and placement of the handle. It sits too close to where you’d mount optics, and it’s easy to rip some fingers if you don’t use an aftermarket handle.

The pencil barrel and lack of viable replacement options gets stuck in our craw. Also, the hooded front sight is too large and distracting; the rifle is better off without it. Frankly, most of our complaints are addressed by the never-released FNAC.

Our largest dislike is the lack of available OEM parts, and price points are extremely high when you can scour the internet for them.

• SCAR 20S: A near 1:1 clone of the Mk20S.

Remington ACR

Overview

The Adaptive Combat Rifle (ACR) is a modular piston-driven rifle that was submitted as a contender to replace the M4. Although designed to be extremely modular with many improvements over the AR-15, the ACR platform never took off — likely due to high cost and lack of support.

Design

The ACR was developed by Magpul Industries as the Masada in 2007. Two years later, Bushmaster brought the rifle to the civilian market as the Adaptive Combat Rifle (ACR). Bushmaster’s ACR was very similar in design and appearance to the original Magpul design; however, engineering and internal design changes were made to improve the reliability of the rifle and make it easier to manufacture.

In 2010, Remington, a sister company of Bushmaster, developed a unique version of the ACR, submitting it as an entry for the Individual Carbine competition. Remington’s ACR featured a magnesium lower receiver instead of the polymer receiver of the ACR/Masada. Remington also made other changes, replacing the lever under the handguard, allowing for quick barrel changes with a standard barrel nut to save weight, including lower-profile folding charging handle and replacing the folding buttstock with a stronger non-folding buttstock (although it was still adjustable for length and comb height).

What We Like

The ACR uses a cleaner (and cooler) piston system. Operating the charging handle on an AR-15 requires the shooter to move their head, whereas the ACR’s bolt handle is easily operated by the support hand while still in a firing position.

Locking the bolt to the rear on an AR-15 is a bit of a three-handed affair (especially while trying to clear a malfunction), while the ACR’s charging handle placement up front and the bolt catch and release located at the front of the trigger guard makes it very intuitive.

The AR-15 requires a receiver extension in order to operate; the ACR’s action accommodates a folding buttstock that can be folded while the gun is fired. And, although the AR-15 can be fitted with an ambidextrous safety, the ACR ups the ante with an ambidextrous mag release, bolt catch, and bolt release.

Finally, want to change calibers? The ACR was designed for quick and easy caliber changes — even an option for an AK-style lower (though these were more theoretical products than reality).

What We Don’t

The ACR is front heavy. We get it, you’re big and strong and don’t care about a front-heavy rifle. But think about it this way: No matter how far you can carry a heavy rifle or how fast you can shoot and move with it, you’ll be able to carry a lighter rifle even further and move even faster.

When the weight is up front, like the ACR, it feels sluggish and we notice fatigue on the range faster. The Remington ACR mitigates the weight issue a bit by removing the quick-detach barrel handle and using magnesium for the receiver; however, it’s still noticeably heavier to hold in a shooting position than a similarly equipped AR-15.

Also, even though the rifle was designed with extreme modularity and despite all of Bushmaster’s promises over the years, the ACR was never truly supported as a platform. Although the Remington ACR can use standard AR-15 grips, you’re stuck with the grip that comes as part of the receiver for the Bushmaster ACR.

As far as we know, there are only 12 Remington ACRs in existence, and all Bushmaster products were discontinued earlier this year.

CZ BREN 2 Ms

Overview

OK, we admit it. This one’s currently a favorite among the collection of non-AR 5.56 rifles; it checks all the boxes we like to see in a next-gen carbine. Soft-shooting, reliable (in the 16-inch barreled variant at least), great ergos, and with the beginnings of aftermarket support, the BREN 2 Ms/806 may well be a certified SCAR killer.

Design

Its antecedent, the 805, was built for the Czech government, which set out a set of criteria for their vision of a new service rifle. CZ took that design and sent it to fat camp, incorporating more polymer into its architecture and shedding over a pound in overall weight — without resorting to tricks like a pencil-profile barrel. It was developed from the outset to be compatible with the 7.62×39 cartridge, so it’s overbuilt for 5.56 and offers a more robust locking system in its multi-lug bolt.

What We Like

The CZ’s side folding stock is similar in features to the ACR’s stock, but its carbon-fiber reinforced polymer appears to be more robust. We’ve run over 2,000 rounds through our example without cleaning, and it shows no sign of grunge in the upper receiver and only light carbon buildup on the gas system, which is set up to accommodate suppressor use through a twist of the regulator. The entire rifle is modular and can be configured at the user level with nothing more complicated than a Torx wrench. There’s plenty of rail space to add lights, lasers, UBGLs, or a toaster oven. Its recoil impulse is like shooting a raced-out AR, and it begs to be run fast.

What We Don’t

Although the trigger pull is one of the best we’ve encountered in a service rifle, the stock trigger blade has excessive slop, which can be easily cured by an aftermarket replacement from HB Industries. There have been reports of gas block failures in short barrel variants, indicating there’s still work to be done on port pressures, dwell time, and material selection when the design strays from the relatively benign design parameters of the full-sized gun. But we’re confident CZ will overcome this. Just don’t be a beta tester.

Ah, COVID, why do you screw with us so? The global pandemic has put a dent in the BREN’s roll-out, but in a post-vaccine world, 1,600 bucks should get you access to one of the best piston guns out there.

FAXON ARAK-21

Overview

If you look through archival issues of RECOIL, Faxon’s ARAK-21 shows up several times. Back in RECOIL Issue 7, we had a preproduction rifle we were told would combine the best features of the AR-15 and the AK into a single unit; hence the ARAK-21 name. We reviewed the production gun in 2016, and unlike some of the rifles adorning these pages, Faxon is still selling, supporting, and developing the ARAK.

Design

The ARAK-21 stands out in that it’s available as both a stand-alone rifle and as an upper receiver you can pop on whatever AR lower you please. The charging handle is forward-placed and swappable.

True to the AK-namesake, the ARAK has a long-stroke gas piston that attaches to the bolt carrier itself instead of the now-more-common short-stroke system. The bolt features wave-form lugs that eliminate the stress risers found on the AR-15. It also has a second ejector to ensure reliable function and consistent ejection patterns.

What We Like

Out of all of the rifles with swappable barrels, Faxon has the most factory options. ARAK barrels come in 12.5, 16, and 20 inches, with three profiles and three calibers (5.56mm/7.62×39/300BLK) available directly from Faxon.

Out of the box, the ARAK-21 comes with a four-position gas block, the Goldilocks zone for customizing gas to your setup without spending too much time figuring out the ideal setting. We found that when the ARAK starts running a little sluggish, a quick uptick in the gas keeps her running, even when suppressed and dirty.

What We Don’t

The ARAK-21 suffers from the same ergonomic issue as the Strike Industries SIAR (see page 86) when you try to lock the bolt to the rear using a standard AR lower. It can become a juggling fumblef***, as you need to press the bolt catch while pulling the charging handle to the rear. However, this can be solved with an advanced lower that allows you to depress the bolt catch with your firing hand.

ROBINSON ARMAMENT XCR

Overview

Robinson Armament submitted the XCR alongside the SCAR as a competitor for the SOF Combat Assault Rifle program (hence the SCAR’s namesake). It too is a piston-driven carbine designed as a highly reliable and flexible duty rifle, suited for a multitude of environments and applications. While it didn’t make the cut for SOCOM, we’ve heard it’s been adopted by some law enforcement agencies in the U.S.

Design

The XCR features a proprietary bolt with wider, thicker lugs than those found on AR-pattern bolts, with a straight tail and an overall general shape similar to that of an AK. One of its distinguishing design features is a long-stroke piston system (also similar to the AK) that has one less moving part and a more gradual recoil impulse when compared to short-stroke counterparts. The gas system is adjustable to compensate for ammunition variance, suppressor use, and different calibers.

On that note, the XCR was also built from the outset to be a multi-caliber system. Calibers and barrel lengths are swappable by the end user. Currently, Robinson Arms offers conversion sets in 5.56, 7.62×39, 300BLK, 6.8SPC, and .224 Valkyrie. A 5.45×39 was offered for some time, then withdrawn, but rumors persist that it may be reintroduced. For those who desire more punch, the larger XCR-M is available in .308 with multiple barrel lengths.

The non-reciprocating charging handle is located on the left side of the receiver, featuring a built-in forward assist feature. All other fire controls are ambidextrous.

What We Like

The XCR-L is nimble in the hands and feels lighter than it is. The use of an adjustable long-stroke piston results in an incredibly soft-shooting rifle that can be rearranged into a wide variety of barrel lengths and calibers, depending on desired application. While some of the caliber choices are more available, and more reasonable, than others, we applaud this feature being both baked into the design and supported by the manufacturer. Also, if you’re a die-hard fan of AR-style stock options, there’s a buffer-tube adapter that’ll allow you to swap rear-ends ad infinitum as desired.

What We Don’t

With all of the other controls being ambi, the need for left-handed shooters to reach over or under the gun to run the charging handle gnaws at us as a suboptimal design. 

Unlike the FN SCAR, OEM parts appear to be readily available on Robinson’s website, and easy-to-use drop-down menus make ordering caliber conversions relatively simple. But the XCR aftermarket is non-existent, leaving you at the mercy of what the manufacturer deems necessary or desirable in terms of parts, features, and configuration options.

SIG SAUER MCX / MCX VIRTUS

Overview

Though SIG Sauer isn’t new to the world of rifles by a long shot, the MCX was their first push toward a truly modular rifle with an AR-ish base. First released in 2015, it was shortly followed up with what could be called the MCX 2.0: the MCX Virtus. Though they’re different rifles, they share the same basic operating system, close enough that we list them here together.

Design

For the MCX, SIG examined the downsides of the AR and modernized it using features present on many other rifles, integrating them all in a holistic way. Like every other rifle in this column, the MCX and MCX Virtus are gas piston guns. Unlike every other rifle in this column, though, the MCX and MCX Virtus use an AR-style charging handle to closely match the AR’s manual of arms. MCX uppers can be installed on a standard AR lower with a conversion, but it’s more at-home on its native receiver complete with integral QD cups.

The MCX’s bolt arrangement and gas system are reminiscent of the AR-180, especially with the dual internal recoil springs. Its gas regulator allows for suppressed use without over-gassing, and the barrel can be replaced with a simple torque wrench. The wearing parts of the upper, such as the brass deflector, internal rail, and charging handle catch, are user replaceable.

What We Like

SIG went whole hog into the MCX Virtus and have now built out an entire family of weapons. All told, there are more than 500 possible combinations you can put together, with more coming at a rapid rate. We weren’t shocked to learn some SOCOM units began fielding them almost immediately.

While current caliber offerings include 5.56 and 300BLK, by the time of the publication, 7.62×39 models and conversions may be available.

What We Don’t

Much to the chagrin of OG MCX owners, although the MCX Virtus looks similar, not all parts are backward compatible. This wasn’t SIG’s initial intention, but they ultimately decided the increase in modularity and reliability of the MCX Virtus design over the MCX warranted the decision. Rails are not backwards compatible, and bolts must be matched to their corresponding generation of barrels. This is due to the tapered bolt lugs on the new generation, but here's the good news: barrels ship with their appropriate bolts. 

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