The Maxim Defense PDX Upper in .300 Blackout (Maxim Defense part no. MXM-47827) is a 5.5-inch-barrel, 14-inch-overall upper receiver group built for close-quarters and personal-defense AR builds where every inch of length matters. Maxim designed the PDX around its HATEBRAKE muzzle booster, which tames recoil and flash while pushing gas and concussion downrange away from the shooter, running on a reliable direct impingement system. It installs on any mil-spec AR-15 lower receiver, and this listing is the upper receiver group only, so a compatible lower and stock or brace are needed to complete the build. Because the finished configuration determines its legal status, review the compliance note below before assembling it.

Key Specifications

• Chambered in .300 Blackout.
• A 5.5-inch barrel.
• 14 inches overall length as an assembled upper.
• Installs on any mil-spec AR-15 lower receiver.
• Includes Maxim’s HATEBRAKE muzzle booster, which reduces recoil, cuts flash signature, and directs gas and concussion downrange.
• Runs on a direct impingement operating system.
• Sold as an upper receiver group only; no lower receiver, stock, or brace is included.

Frequently Asked Questions

What lowers does this upper fit? It’s designed to install on any mil-spec AR-15 lower receiver.

Does this come with a lower, stock, or brace? No. This is the upper receiver group only; a lower receiver and a stock or brace are sold separately.

Since the barrel is under 16 inches, is this an NFA item? Not by itself. A bare upper without a lower receiver isn’t a regulated firearm under federal law. Whether the finished gun is a legal pistol or a Short-Barreled Rifle depends entirely on the lower and configuration it’s built on: paired with an AR pistol lower and an arm brace (not a shouldered stock), it builds into a legal AR pistol with no NFA paperwork. Putting it on a shouldered rifle lower makes a short-barreled rifle; since October 9, 2026, ATF no longer requires Form 1 approval or registration for that, but state law still applies, and taking an SBR across state lines needs an approved ATF Form 5320.20. Always confirm your lower’s configuration and current registration status before assembly.

Buy the Maxim Defense PDX Upper, .300 Blackout

Confirm your lower’s configuration before you build, then order this upper from our Lubbock, Texas gun shop for fast, insured shipping to your door. Shop the full Maxim Defense Industries lineup or browse our complete AR15 uppers selection.