Maxim Defense MDX:505 SBR .300 AAC Blackout, 5.5 in Barrel, Two-Tone Tan, AR-15 Platform
This Maxim Defense MDX:505 is a factory short-barreled rifle in .300 AAC Blackout with a 5.5-inch barrel and a compact AR-15 layout. It features billet 7075-T6 aluminum receivers, a free-float M-LOK handguard, and a two-tone earth/tan anodized finish. This NFA-regulated short-barreled rifle comes with low-profile folding sights installed and quality controls including an ambidextrous safety and a Radian Raptor-LT charging handle.
Condition
Overall Condition: Excellent condition, showing only faint signs of any prior use or handling.
Bore Overall Condition: Excellent - Clean, mirror finish, no signs of wear.
Bore Rifling: Excellent Rifling - Sharp lands and grooves.
Specific Condition Details: This rifle shows minimal markings from previous handling and use. Please refer to the photos for additional detail.
What’s Included
- Maxim Defense MDX/PDX owner's manual
- Maxim Defense M-LOK handguard (spare, not installed)
- Maxim Defense factory box
- 1x 20-round Lancer L5-AWM magazine
- Folding iron sights (installed)
- Radian Raptor-LT ambidextrous charging handle (installed)
- M-LOK vertical foregrip (installed)
Built on billet 7075-T6 aluminum upper and lower receivers with a two-tone earth/tan anodized finish, the MDX:505 delivers a durable, compact .300 BLK SBR package. The 5.5-inch barrel configuration achieves an approximate 18.75-inch overall length when collapsed (per manufacturer data), allowing a small footprint while retaining full rifle features. This is an NFA-regulated short-barreled rifle.
The free-float 6061-T6 aluminum handguard provides M-LOK slots at 3, 6, and 9 o'clock and integrates QD sling sockets. A continuous 12 o'clock Picatinny rail runs across the receiver and handguard. The rifle includes a forward assist, shell deflector, and ejection-port cover. The lower receiver bears the manufacturer marking "MAXIM DEFENSE INDUSTRIES, ST. CLOUD, MN USA," while the upper displays a "300 BLK" caliber marking and a Maxim Defense logo near the handguard interface.
Controls are purpose-built: an ambidextrous safety selector, a Radian Raptor-LT ambidextrous charging handle, and a standard magazine release. The PDW-style telescoping stock uses dual guide rods, a skeletonized buttplate, and a hex-textured pad. Low-profile folding iron sights are installed, and the threaded barrel wears an open-prong flash hider.
As configured, this AR-15 platform SBR runs a semi-automatic direct-impingement gas system in .300 AAC Blackout with a 20+1 capacity using the included Lancer L5-AWM 20-round magazine. An M-LOK vertical foregrip is installed, and a spare Maxim Defense M-LOK handguard is included along with the factory box and MDX/PDX manual.