While the MP5, HK33 and G3 are still very effective platforms, firearms have gained innovations over the past half century that don’t easily fit within the constraints of these legacy designs. Modern manufacturing techniques mean that machined billet aluminum alloy is lighter and more affordable than steel stampings. Modularity can be built-in, rather than requiring some type of adapting interface for mounting optics and other accessories, allowing a firearm to be tailor fit to the shooter. The market is dominated by certain patterns of furniture, magazines and fire control components, meaning that compatibility with these formats allows for the end user to easily upgrade and customize their own firearms. Zenith Firearms has attempted to offer the best of both worlds—a traditional roller-delayed blowback operating system in a firearm with the latest 21st century features with its Modern Roller-Delayed System (MRDS).
Zenith got its start importing Turkish-made guns including license-built versions of Heckler & Koch designs. The company transitioned to building firearms in-house at its Afton, Va. design and manufacturing facility, currently offering MP5, HK33, G3 and AR-15-type firearms. In 2018, Zenith submitted its Z-5 design to the U.S. Army’s Sub-Compact Weapons competition. Lessons learned from that experience are incorporated into the MRDS.
At first glance, MRDS firearms look distinctively “Modern Sporting Rifle.” Both upper and lower are machined from billet aluminum alloy. The lower follows a basic AR-profile, with a generous magwell and removable trigger guard. The fire control unit is AR-15 compatible, as is the removable pistol grip. MRDS firearms fly their "Made In America" banner proudly through the use of a Team Accessories Corp. (TAC) polymer pistol grip with a rubber flag overmold. Controls are fully ambidextrous with a bilateral safety lever, bolt release and magazine release in positions that will be familiar to AR users. Magazines are also AR-compatible, allowing for the bolt hold-open feature, and an Amend2 Mod. 3 30-round magazine is supplied for 5.56 NATO and .300 Blackout models.
The upper receiver has bolted-in steel guide rails for the bolt carrier and a steel trunnion that engages the bolt’s rollers. A spring-loaded dustcover closes the ejection port and opens automatically when the bolt moves, like an AR-15. Both the dustcover and shell deflector are polymer, with the latter being replaceable. Although the recoil springs are contained with the receiver, an M4-style receiver extension is mounted from the receiver’s end cap (held in place by two H&K-style push pins) to allow for the mounting of telescoping M4-style stocks. An Amend2 Modular Stock (AMS), which includes sling loops, QD swivel spigots and a rubber recoil pad, is supplied on firearms sold in rifle configurations.
The oval-profile aluminum alloy handguard mounts to the receiver and allows the barrel to free float. It has a built-in guide for the forward-mounted charging handle, meaning there is no cocking tube housing that needs to be attached to the barrel, like traditional H&K-style firearms. In fact, due to the roller-delayed blowback mechanism’s lack of a gas system, there is nothing touching the barrel from the point it leaves the trunnion. The handguard has M-Lok slots in the 2, 3, 4, 6, 7, 8 and 9 o’clock positions, in addition to a section of M1913 Picatinny rail in the 12 o’clock position, and a QD sling swivel spigot on each side. It mates to the rail section machined into the top of the receiver for 19.25” of real estate for mounting optics or other accessories in 14.5” and 16”-barrel versions.
Where the MRDS stays the closest to legacy designs is in its roller-delayed blowback operating system. While there are other manufacturers offering their own unique roller-delayed firearms these days, there’s nothing quite like the original, dual horizontal roller design pioneered in World War II that results in a lightweight bolt and carrier and renders that legendary smooth recoil impulse. The bolt carrier has serrations meant to give traction for the thumb to “forward assist” the bolt, as found on some G3 rifles. The forward charging handle is a folding and extended G3-style unit that can be locked into its rearward position to hold the bolt open or charge the firearm with a classic H&K slap. In current production models, the charging handle can be moved to the right or left side, and a replaceable steel charging handle notch is incorporated into the aluminum handguard.
Zenith’s MRDS platform is currently available in four chamberings, including 5.56 NATO (ZF-56), .300 Blackout (ZF-300) and 7.62x39 mm (ZF-39). These are available in stocked 14.5” (all NFA rules apply) and 16” barrel rifle configurations, as well as stockless pistols with barrel options between 7.5” and 14.5”. To showcase the potential for the design outside the competition, recreational shooting and law enforcement/self-defense realms, Zenith makes the ZF-350. The rifle, chambered in 350 Legend with a 20” barrel, exhibits the ability for the MRDS system to be used for hunting rifles chambered in today’s AR-15-size straight-wall cartridges. MSRPs for these firearms range between $1,669 and $1,950.
To evaluate the platform, we tested an early rifle version of the ZF-56 with a 16” barrel. It uses a lightweight barrel, just slightly larger in diameter than an A1 or “pencil” profile, from the chamber forward. The muzzle of the barrel is threaded 1/2x28 TPI and fitted with an A2-style muzzle brake. The rifle weighs 7 lbs., 14 ozs. No iron sights are supplied, so we paired the ZF-56 with a Swampfox Arrowhead 1-10X LPVO mounted with the company’s Hostile Engagement cantilever mount.
The rifle’s manual of arms, combining features of both AR-15 and H&K controls, work well together. It would be easy for a user of either platform to transition to an MRDS firearm.
Reliability was 100 percent with a wide selection of both brass and steel-cased ammunition. As is common with roller-delayed blowback firearms chambered in rifle cartridges with fluted chambers, brass cases exhibited flow marks and got a nasty dent from the case deflector that would preclude them from being reloaded. The rifle’s single-stage trigger breaks crisply with minimal take-up at just under 6 lbs. and resets positively.
Take down and maintenance are equally simple and intuitive. In AR-15 fashion, two captured takedown pins hold the upper and lower receivers together. The bolt carrier group and springs are removed through the rear of the upper receiver. Bolt and carrier disassemble just like an H&K. A added benefit of the roller-delayed blowback system? There is no gas system to disassemble or clean. To round out the package, the ZF-56 we tested shipped in a Zenith-branded, TSA-compliant, lockable hardcase—something that should be more common in the world of $2,000 rifles.
Zenith’s current offerings are not the limits of the MRDS platform. The company recently announced “PDW” MRDS firearms designed for handgun cartridges. The ZR-9 and ZR-10 are chambered in 9 mm Luger and 10 mm Auto, respectively, and feed from Glock-compatible magazines. The 9 mm model will be available with 6” and 9” barrels and weigh 4 lbs., while the 10 mm version will have a 7.5” barrel and weigh 4.5 lbs.
Beyond their chamberings, these firearms follow the pattern of their rifle-cartridge siblings. Barrels are cold hammer-forged with both a three-lug adapter and threading on their muzzles (1/2x28 TPI for 9 mm and 9/16x24 TPI for the 10 mm). The pistol-caliber MRDS firearms are scheduled to be released at SHOT Show 2026 and their MSRP has not been announced. Zenith has also discussed the possibility of a “large frame” version of the MRDS, scaled for 7.62 NATO-class cartridges, in the future.
By introducing its Modern Roller-Delayed System, Zenith Firearms is offering all the timeless benefits of the roller-delayed blowback system without having to compromise on the features that modern shooters expect. The result is modular, optics-ready and AR-15-compatible firearms that stand out from the MSR herd.
