Diamondback DBX57 5.7x28mm Pistol: Full Review

What makes a good personal defense weapon (PDW)? Ideally, a good PDW packs firepower into a compact package. The HK MP5K was likely the first submachine gun marketed as a “PDW,” and this type of firearm was developed for defensive security. Commercial versions of the select-fire, pistol-caliber subguns have increased in popularity, which we’ve seen with products including the Maxim Defense PDX ($2,295); SIG Sauer MPX K ($2,298) and the discontinued Springfield Armory Saint Edge PDW. Some models such as the SIG Sauer MCX Rattler ($2,099) are offered in .300 Blackout or 5.56 NATO cartridges.

PDWs are practical for vehicle use, concealed carry in backpacks and certain everyday carry situations. They are easy to wield in close quarters, but the carbine cartridges can ballistically suffer from reduced velocity while producing nasty muzzle blast.

The 5.7x28mm, however, does not. The 5.7 cartridge has been around since the FN P90 and Five-seveN pistol were introduced in 1990s. It is an intermediate cartridge that bridges the gap between rifle and pistol cartridges. Agencies around the world have used it effectively since. In fact, in the U.S., the 5.7x28mm is enjoying a revival thanks, in large part to new firearms such as the Ruger-5.7 ($869).

Until now, the civilian-legal PS90 ($1,949) has been the prevailing PDW chambered in the 5.7mm cartridge. It’s about to get some competition now that Diamondback has released its DBX57. It’s an AR-style pistol that brings affordable familiarity to the PDW.

The Standouts

The first feature that stands out to a user about the DBX57 is its profile. It is very slim, measuring just 1 3/4 inches at the widest point. In fact, it is just as slim as many modern concealed-carry pistols at the upper receiver and handguard! The DBX is balanced well, too. It’s hard to believe that it even weighs 3 pounds when unloaded. That is lighter than perhaps every PDW on the market today.

The DBX has an 8-inch barrel complete with 1/2x28 threads at the muzzle. Diamondback even designed its own flash hider for the DBX.

The reciprocating charging handle is ambidextrous and there is a Picatinny rail section at the rear to attach a folding pistol brace.

The trigger, safety and handguard are all AR-15 compatible, which means that they’re easy to change. However, left-folding arm braces will obstruct the safety selector, so installing an ambidextrous selector lever is a good idea in case you needed to shoot the DBX57 with a folded brace.

The biggest differentiator in familiarity between the lower receiver on an AR-15 and the DBX57 is the bolt catch. Still, the DBX57 offers a last-round bolt-hold-open function, but unlike an AR, the bolt catch needs to be pressed down to close the bolt. This is easy to account for if you’re familiar with handling a DBX, but it’s important to note and practice using at the range before you carry it.

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The continuous top rail is Picatinny spec and perfect for attaching reflex sights, or a red dot paired with a magnifier. It’s just long enough to provide an effective sight radius for backup sights. There are also plenty of M-Lok slots for mounting forend lights, lasers and angled foregrips, too.

Internal Dialogue

While the DBX looks and handles something like an AR-15, its function is unique. The DBX operates from a dual-piston system, which can be adjusted without needing to disassemble the gun. Just use a flathead screwdriver to tune the gas block to the load. Ports on the left and right side of the handguard reveal four numbered settings, and it’s important that they match for proper function.

This is significant for both suppressed and unsuppressed shooting. PDWs chambered for 5.56 NATO, .300 Blackout and 9mm tend to spit a lot of gasses toward the shooter when suppressed. Reliability can also be finicky.

The gas system on the DBX is going to send most of the gas away from the shooter, and the easy-to-adjust pistons could support suppressed and unsuppressed shooting. The numbered settings also afford the ability to keep several different cartridges on-hand; record the best setting for each cartridge in order to change ammunition quicker.

Gas-piston-operated systems also run cleaner than direct-impingement systems. End-users will appreciate this when they first take the DBX57 apart.

The DBX57’s unique operation does make it a challenge to disassemble and reassemble — and it is almost impossible to take apart without a hammer and punch. While the lower receiver looks like an AR, the pressure on the takedown pins reminds us that the internal function is not standard AR. Disassembly is complicated enough that few people will do it without instructions. Fortunately, Diamondback Firearms includes highly detailed manual, and there is also a video instruction online. It only takes a few times to learn it, but don’t expect to fieldstrip the DBX as fast as an AR.

Performance

As one could expect when shooting a 5.7-chambered firearm, the Diamondback DBX was a pleasure to shoot. Even for weighing 3 pounds, recoil was like shooting a .22, and there are a few different ways to shoot the DBX57.

The most obvious configuration is to add a folding arm brace and shoot it like an AR pistol. However, the Magpul MOE K pistol grip is at an angle that’s optimal for shooting the DBX57 like a standard full-size pistol. Yes, it’s heavier than most striker-fired handguns, but it’s not so heavy that it’s impractical. Last, it’s easy to shoot accurately to 25 yards with a sling or with a folded brace from inside a tight spaces, especially when using a red dot.

Driving this pistol to a target is fast, and transitions are quick. Follow-up shots are a breeze. A red-dot sight with backup irons would be an effective EDC sighting solution. A small, low-powered variable optic (LPVO) would be appropriate, too, but more than a 1-6X is too much. The DBX57 is not precise enough for more scope.

While zeroing at 50 yards, most groups printed between 1 and 3 inches. This is perfectly acceptable for close-quarters personal defense, and groups didn’t open up much at 100 yards. Beyond 100 yards, five-shot groups were more than 3 inches.

There was considerable variance in velocity between the different cartridges. The DBX preferred Federal’s 40-grain American Eagle load for reliability. This round also happens to be the most prevalent and affordable option.

Vanguard Outfitters’ 34-grain cartridge was the fastest, but it also presented the most feeding issues, likely due to the unique shape of the bullet; it would often chamber improperly.

For a defensive round, the DBX57 had no feeding issues using the R&R Weapon Systems 40-grain, fragmenting copper hollow point.

Elite Ammunition’s ammo also did well, but it was occasionally necessary to give the charging handle an additional tap to ensure it was in battery.

Fitting the Bill

What makes the Diamondback DBX57 most interesting to us is that it brings the 5.7x28mm cartridge into a slim, practical and affordable PDW platform. It is one of the smallest and lightest PDWs on the market, and it offers recoil-friendly firepower without the muzzle blast of a short-barrel AR in 5.56 NATO or .300 BLK. It’s modular and easy to shoot like an AR, and it’s reliable enough to carry with confidence. Our biggest complaint was that it doesn’t come with an arm brace. A folding brace would put the DBX57 on par with the other popular PDWs.

The DBX57 has the quality and features that call attention to the parent company. Diamondback Firearms have shown that they are capable of exceptional engineering and manufacturing. PDWs are one of the greatest tools in a self-defense arsenal, and Diamondback’s DBX57 wouldn’t take up much space in yours.

Diamondback DBX57

• Type: Gas-piston operated, semiautomatic
• Cartridge: 5.7x28mm
• Capacity: 20+1 rds.
• Barrel: 8 in., 1:9-in. twist
• Overall Length: 15.25 in.
• Weight: 3 lbs.
• Grip: Magpul MOE K
• Finish: Hardcoat anodized
• Trigger: 6 lbs.,14 oz. (tested)
• Safety: Two-position selector
• Sights: None
• MSRP: $1,352
• Manufacturer: Diamondback Firearms, 321-325-5995, diamondbackfirearms.com