Maxim Defense Industries PDX, Complete Upper, 300 Blackout, 5.5″ Barrel, Black Finish

Product Description

The Maxim Defense Industries PDX complete upper in 300 Blackout features a 5.5″ barrel and black finish, designed for close-quarters applications where size and maneuverability matter most. This is a drop-in upper assembly for AR-15 pattern lower receivers. The PDX platform is engineered for shooters who need a compact rifle configuration without building from individual components.

The 300 Blackout chambering allows subsonic and supersonic ammunition interchangeability. Short barrel length makes this upper well-suited for confined spaces, vehicle work, and home defense scenarios where a full-length rifle is impractical.

Key Facts

• Model: Maxim Defense Industries PDX Complete Upper
• Caliber: 300 Blackout
• Barrel Length: 5.5″
• Finish: Black
• SKU: MXM-47827

Why 300 Blackout in a Short Barrel Configuration

The 300 Blackout cartridge was designed specifically to perform in short barrels. Unlike 5.56 NATO, which loses significant velocity and effectiveness below 10.5 inches, 300 Blackout maintains reliable powder burn and ballistic performance in compact configurations. A 5.5″ barrel extracts nearly full potential from the cartridge when firing subsonic loads, and still delivers effective terminal performance with supersonic ammunition.

This chambering also enables suppressor-friendly operation when paired with subsonic ammunition. The cartridge uses standard AR-15 magazines and bolts, requiring only a barrel swap to convert an existing rifle platform. Shooters gain the ability to run heavy projectiles—often 200+ grains subsonic—through the same magazine footprint as a standard AR.

What to Pair With the PDX Upper

This complete upper installs on any mil-spec AR-15 lower receiver. You will need a lower that is either registered as a pistol or configured as an SBR with an approved ATF Form 1, since the barrel length is under 16 inches. Installing this on a standard rifle lower without NFA approval creates an illegal short-barreled rifle.

Optic selection should account for the short sight radius and close-range role. Red dots with 2 MOA or larger reticles, holographic sights, and low-power variable optics are common choices. The compact envelope also benefits from a muzzle device that manages concussion, as short barrels in 300 Blackout produce sharp muzzle blast.

For questions on compatibility with your current lower or build plans, call our shop at 817-393-4000. We can confirm headspacing, recommend complementary components, and discuss your intended application.

Compact vs. Full-Length 300 Blackout Uppers

The tradeoff between a 5.5″ barrel and a traditional 10.5″ or 16″ configuration centers on portability versus velocity. Supersonic 300 Blackout ammunition loses roughly 100-150 feet per second for every inch of barrel removed below 10 inches, which reduces effective range. However, subsonic loads—the ones designed for suppressed use—show minimal velocity loss in short barrels because the powder charge is already tailored for low-speed operation.

If your primary use case involves suppressed subsonic shooting inside 100 yards, the compact PDX upper sacrifices little real-world performance. If you plan to shoot supersonic hunting or defensive loads at extended distances, a longer barrel recovers some downrange energy. Consider how you will actually use the rifle, and whether the reduced length matters more than the marginal ballistic advantage of added inches.

Buying from Western Sport in Roanoke, TX

Western Sport is a licensed FFL/SOT dealer serving North Texas and the DFW area since 2007. We handle transfers, special orders, and provide on-site gunsmithing services under the direction of Lance Rankin. Whether you need assistance with a build, a transfer, or product recommendations, call us at 817-393-4000. Our shop is located in Roanoke, Texas.

Frequently Asked Questions

Q: Does the Maxim Defense PDX upper require an NFA tax stamp?

A: The upper itself is not a regulated item, but installing it on an AR lower creates a configuration subject to NFA rules because the barrel is under 16 inches. If your lower is registered as a pistol, no additional tax stamp is required. If your lower is a rifle, you must file ATF Form 1 and receive approval before assembling the two components, or you will have created an unregistered short-barreled rifle.

Q: Will this PDX upper fit my AR-15 lower receiver?

A: The Maxim Defense PDX upper is designed to fit mil-spec AR-15 lower receivers. It uses standard takedown and pivot pin spacing. If your lower has non-standard dimensions or proprietary features, call 817-393-4000 so we can confirm compatibility before you order.

Q: Can I shoot both supersonic and subsonic 300 Blackout through this upper?

A: Yes, 300 Blackout was designed to cycle both supersonic and subsonic ammunition without requiring gas system adjustment in most configurations. You can switch between load types using the same magazines and bolt. Performance characteristics will differ—supersonic loads produce higher velocity and flatter trajectory, while subsonic loads remain below the speed of sound and are quieter when suppressed.

Q: What is the advantage of a 5.5″ barrel in 300 Blackout over 5.56 NATO?

A: The 300 Blackout cartridge achieves more complete powder burn in short barrels compared to 5.56 NATO. A 5.56 round loses substantial velocity and produces excessive muzzle blast when fired from a barrel under 10 inches, because the powder is still burning outside the muzzle. The 300 Blackout uses faster-burning powder tailored to shorter barrels, so you retain more usable energy and reduce wasted combustion.

Q: How do I transfer this upper through Western Sport?

A: Complete AR uppers do not require an FFL transfer because they are not firearms under federal law—the lower receiver is the regulated component. You may order this upper for direct shipment or pick it up at our Roanoke location. If you have questions about your specific situation or need assistance with a complete rifle transfer, call 817-393-4000.

Q: What optic height should I use on the PDX upper?

A: Optic mount height depends on your shooting position, whether you use a suppressor, and personal preference for cheek weld. Lower-third co-witness mounts are common on compact uppers because they allow a natural head position without adding excessive height over bore. Absolute co-witness and taller mounts are also viable depending on your setup. Call 817-393-4000 to discuss your intended configuration and we can recommend mounting solutions that work with the PDX platform.

Additional information