U.S.A. -(AmmoLand.com)- Full disclosure, I’ve been a naysayer of 5.7x28mm firearms like the Diamondback DBX for years. To be honest, I never saw the appeal of the round – it seemed like an over-priced 22 Magnum.
But the round’s lightweight design and ultra-fast velocity did pique my interest when last year’s SHOT Show saw the release of several new guns in the caliber. The majority of them seemed to play it very safe with their guns, with most being modifications of existing designs. But one company decided to release something no one was expecting.
Enter, the Diamondback DBX
The Diamondback DBX is a semi-automatic, magazine-fed piston-driven firearm that looks like the result of crossbreeding an FN SCAR with an old Walther MPL submachinegun. But it’s what’s under the hood that really matters in this case.
The majority of large-format pistol-caliber firearms operate via direct blowback. Meaning the action doesn’t lock, and the bolt is only held closed by the tension of the recoil spring. To ensure the recoil of the action isn’t too violent or premature, these firearms tend to use heavy bolts as a way of delaying the cycling of the bolt.
This is done primarily for two reasons: First, it works. It’s very simple, and relatively easy to manufacture, and even if the gun gets congested with carbon or debris, it will continue to function reliably. The other reason is cost. Direct blowback bolts and carriers do not require as much machining time as locked-breech designs simply because no precise camming surfaces are necessary. Another cost-saving aspect of this is that these guns normally don’t have a gas tube and utilize the barrel itself to transfer some of the expanding gasses rearward to cycle the action.
The downsides of this method of operation are weight and disproportionate recoil. With a heavy bolt and stiff recoil spring, the reciprocating mass of the carrier group tends to amplify the recoil impulse as it moves towards the rear of the gun. With shoulder-fired weapons, this isn’t as large of a concern. Still, pocket pistols like Walther’s PPK can be relatively snappy, and the stiff recoil spring makes the slide difficult to rack for shooters with limited upper body strength or arthritis.
Under The Hood
The Diamondback DBX takes a page from the SIG MPX’s playbook and utilizes a short-stroke piston instead. Meaning a hole in the barrel siphons excess hot gas from a detonated round that drives a small piston against the bolt carrier group to cycle the action. (Similar to an MCX, MPX, or Russian SKS)
At the beginning of the recoil cycle, the bolt rotates a small amount, and its lugs cam against a matching recess in the chamber. Once it does so, the action is unlocked, and the bolt carrier group freely travels back against the recoil spring. Once fully back, the compressed recoil spring propels the carrier group forward chambering a fresh round, priming the hammer, and locking the bolt inside the chamber.
Sounds complicated, and in some ways it is, but the result is an incredibly soft-shooting firearm.
Neat, but so what? Well, this is an already uncommon setup for pistol caliber firearms, but exceptionally so for those chambered in 5.7mm; For several years every gun chambered in 5.7mm was exclusively direct blowback. But the recoil reduction and novelty of this aren’t enough to sell most shooters on the gun alone, what about features?
The DBX is loaded with several desirable features that demonstrate that the engineers are Diamondback weren’t simply content to churn out a novelty gun in a semi-niche caliber. There are so many, that we’ll just start at the muzzle and work our way back.
First off, the muzzle features a combination ported brake, flash suppressor, that reduces the already tame recoil of the round to an absurd level. Double and trip taps on targets are a breeze with the recoil of the DBX feeling just slightly more intense than that of a .22lr carbine.
Under this muzzle device, the barrel is threaded to standard 1/2×28 thread pitch, so shooters can install whatever AR-15 muzzle device they want – including sound suppressors. Speaking of which, while 5.7mm isn’t a great round to suppress since it relies on velocity over projectile mass, most quality .22lr rimfire suppressors are designed to handle it. Meaning shooters who already own a rimfire suppressor can get more use out of their tiny, lightweight can. (Note: Not every 22lr suppressor can handle 5.7x28mm, check with the manufacturer before installing one.)
The barrel itself is stainless and features a medium contour that strikes the perfect balance between consistent accuracy and weight distribution. Continuing back, the DBX’s barrel features an adjustable gas system that allows shooters to dial in the gun to their favorite suppressor allowing for the quietest operation possible. This can also be used to increase backpressure should the gun become fouled up or underpowered ammo is used.
Handguard and Upper Receiver
Around the barrel, the Diamondback DBX features an extruded-aluminum free-floated two-piece handguard. The lower portion features two arrays of M-Lok slots on the left and right, with a single set of M-Lok slots on the bottom.
The upper portion of the rail is actually an extension of the upper receiver itself and appears to be milled from a solid block of aluminum. This gives shooters a rock-solid monolith upper Picatinny rail for mounting optics, lights, or lasers that is certain to hold zero.
A few inches back, on the left side of the receiver, the DBX features a combination bolt release/bolt catch. Behind this, it features a skeletonized reciprocating charging handle that is slightly canted downwards to prevent it from snagging on oversized optic mounts. This feature is also great as it keeps shooters from slamming their hands against an optic mount while charging the firearm. Another great feature about the charging handle is that it can be changed from one side to the other, allowing for southpaw shooters to operate the firearm comfortably as well.
The back of the upper receiver features a rounded, ‘hump’ that incorporates a quick detach sling mount which is a very well-thought-out addition, as a single-point sling is ideal for carrying the compact PDW-sized DBX.
Below, the upper receiver attaches to the lower by means of two push-pins just like an AR-15. So field-stripping and cleaning the DBX is a simple, straightforward affair.
The lower takes some obvious inspiration from an AR-15 but is clearly purpose-built for the 5.7mm cartridge and the FN FiveseveN magazine. The magazine well features a scalloped cut for weight reduction and a small amount of flaring to make reloads easier. On the right side, the DBX features an oversized magazine release lever that is situated far enough back that shooters of virtually all sizes can actuate it without shifting their shooting grip.
Inside the receiver, the Diamondback utilizes an AR-15 hammer and trigger components opening up an enormous aftermarket of options for shooters looking to customize their DBX. Further in this vein, the lower receiver incorporates a small Picatinny rail segment at the rear for mounting a stabilizing brace, or even shooters have the proper tax stamp, a stock. The DBX also uses a standard AR-15 pistol grip, but ships with a slim PDW-style Slimline MOE one from Magpul.
For testing, I only had a chance to run 300 rounds of ammo through the DBX due to the current scarcity of ammunition. But in that limited testing, I only encountered a pair of malfunctions stemming from the DBX being run bone dry with a suppressor. Now that’s obviously not how the gun is designed to be run, so why did I do that?
Put simply, if it runs flawlessly with neglect and the increased back-pressure and carbon build-up from a suppressor, the DBX is 100% flawlessly reliable and good to go. Roughly 100 rounds into testing like this, the bolt would occasionally struggle to fully close.
All ammo was Federal FMJ, and the rounds proved exceptionally accurate from the gun’s eight-inch barrel. Out to 85 yards, I never missed an 8-inch target with a simple red dot sight, but I wanted to get a more scientific measurement of the gun’s accuracy.
So I mounted a Nikon M-300BLK 1.5-6x VPO to the DBX and fired it mounted on an old Manfrotto tripod. I had meant to use one of the nice Vortex ones, but the Arca Swiss Rail adapter I ordered hadn’t arrived yet. So I was forced to use an old Magpul adapter that isn’t compatible with the Vortex. Even still, this provided a very solid shooting position for the DBX allowing me to squeeze out nearly every ounce of performance from the gun possible.
Firing from the tripod at 50 yards, the DBX consistently produced 1-1.5in five-shot groups. Occasionally, it would leave one ragged hole for three or more rounds, but on average the accuracy hovered around just over two MOA.
For the most part, everything about the DBX was great, but there were definitely some aspects of the gun that were less than stellar – or at least one in particular: The trigger.
For some reason, the trigger exhibits an old issue that converted military AKs used to have – trigger slap. This is when the trigger slaps the shooter’s finger during the reset. It sounds minor, but in cold weather, it can be really annoying. Not a deal-breaker for me, and I’m sure there’s an easy fix with an aftermarket AR-15 trigger, but I wasn’t going to invest the extra time and effort to troubleshoot a gun I don’t own.
With an MSRP of $1,125, the Diamondback DBX is going to be a tough sell for someone just looking for a fun plinker or a range toy. That said, it’s very well made, incredibly accurate and absurdly light, and handy. These factors, combined with its ability to use FN FiveseveN magazines make it the ideal companion gun to that pistol. To me, this makes the Diamondback DBX a must-have for fans of the 5.7mm cartridge and owners of the FN FiveseveN pistol.
For those folks mentioned above, the DBX will be a home run. For me, I’ll just stick to my boring old NATO calibers.
About Jim Grant
Jim is a freelance writer, editor, and videographer for dozens of publications who loves anything and everything guns. While partial to modern military firearms and their civilian counterparts, he holds a special place in his heart for the greatest battle implement ever devised and other WW2 rifles. When he’s not reviewing guns or shooting for fun and competition, Jim can be found hiking and hunting with his wife Kimberly, and their dog Peanut in the South Carolina low country.