The Diamondback DBX is a pistol-AR chambered in 5.7×28mm — a different category of object than every other handgun-format 5.7 pistol on the market. It uses an 8" threaded barrel, a monolithic 7075-T6 aluminum upper and lower receiver, standard AR-15 fire controls (single-stage Mil-Spec trigger, AR selector for safety), and accepts FN Five-seveN 20-round magazines directly. Diamondback's design pairs a dual gas piston with a locked-breech action — heavier and more complex than the delayed-blowback or rotary-barrel systems in handgun-format 5.7 pistols, but appropriate for the longer barrel and PDW use case.

Weight is the spec that defines the trade-off. At 57.6 oz with the brace installed, the DBX is over four times heavier than the Kel-Tec PR57 and more than double the FN Five-seveN — it is a two-handed, braced shooter, not a sidearm. The reward for that weight is what the 8" barrel does to the 5.7×28mm cartridge: owners report meaningful velocity gains over the 1750-1800 fps that standard FN ammo produces from a 4.8"-5" pistol barrel, in the range of 150-200 fps depending on load. That puts 5.7's terminal performance in a more useful window for anything beyond close-range pistol distances.

The closest comparison in the 5.7 catalog is the Kel-Tec P50, also a PDW-format 5.7 with a long barrel — but the P50 uses the 50-round FN P90 magazine in a bullpup layout. The DBX takes the more familiar AR-15 approach: standard fire controls, Picatinny top rail, M-LOK handguard, and a magazine that drops out the bottom of the lower receiver like a normal AR. Both occupy the same legal pistol classification and stay under the 26" NFA threshold without a tax stamp. The DBX is the choice for buyers who want AR-15 manual of arms in 5.7, with the FN-pattern magazine compatibility most 5.7 owners already have on the shelf.