Maxim Defense MDX:505 .300 BLK AR Pistol, 5.5 Inch Barrel, Arid/FDE, HATEBRAKE, SCW Compact Brace
The Maxim Defense MDX:505 is a compact .300 AAC Blackout AR pistol built for short-barrel performance, featuring a 5.5-inch threaded barrel with the Maxim Defense HATEBRAKE muzzle booster/linear compensator. It uses a slick-side upper and a proprietary SCW compact brace with a shortened buffer system to minimize overall length. Finished in Arid/FDE and made in the USA, it includes one 30-round Magpul PMAG GEN M3 magazine.
Condition
Overall Condition: Good condition, showing some signs of prior use and handling.
Bore Overall Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Notes: The installed handguard and the upper receiver exhibit severe cosmetic damage. Due to the 5.5-inch barrel length, the handguard cannot be swapped for a longer rail. The bolt does not lock to the rear with the current proprietary buffer system; it is unknown whether this is by design for this configuration.
What's Included
- 1x 30-round Magpul PMAG GEN M3 magazine for .300 BLK (polymer)
The MDX:505 pairs a short 5.5-inch barrel with the Maxim Defense HATEBRAKE, a linear compensator and muzzle booster designed to direct blast forward and enhance backpressure in ultra-short setups. The barrel is threaded, offering compatibility with appropriately threaded muzzle devices.
Its slick-side upper omits a forward assist and incorporates an integral brass deflector and ejection-port cover. A compact, free-float aluminum handguard provides a continuous top Picatinny rail, integral QD sling cups, and an underside hand stop for control on a very short front end.
The lower features practical controls and ergonomics, including an ambidextrous safety selector with pictogram markings, a right-side bolt-catch extension lever, a single-stage curved trigger, and a Reptilia pistol grip. The system runs semi-automatic, direct impingement in .300 AAC Blackout.
At the rear, the SCW-pattern compact brace uses dual steel guide rods, a Velcro strap, and a proprietary shortened buffer system to keep length to a minimum. Typical collapsed overall length for the MDX:505 configuration is about 18.75 inches. The pistol wears an Arid/FDE finish on the receiver and brace, with manufacturer markings including "MAXIM DEFENSE INDUSTRIES, ST. CLOUD, MN USA" and "300 BLK" on the upper.