The Maxim SDX RFLX SBR Magpul CTR in Black is the carbine configuration of Maxim’s integrally suppressed .300 Blackout rifle, with an 8.5-inch barrel and a Magpul CTR stock (Maxim part no. MXM-49222). Built on the SDX:508 platform, it uses Maxim’s M-Kore reflex suppressor, a true over-the-barrel design that wraps back over the barrel to reduce added length and improve balance. With a tuned gas system and internal suppressor extension, Maxim rates it at 129 dB at the shooter’s ear with subsonic ammunition. An ALG Combat trigger, a full-length 1913 rail and an M-LOK compatible two-piece handguard complete the rifle. It is a compact suppressed rifle for the truck, the house or the range.

Key Specifications

• The chambering is .300 Blackout, a cartridge built for suppressed use with subsonic loads.
• Barrel length is 8.5 inches, with a 1:7 twist that stabilizes heavy subsonic bullets.
• The stock is the Magpul CTR, Maxim’s carbine option for the SDX RFLX SBR.
• Collapsed length is 32.25 inches with the CTR stock.
• Weight is 7 pounds 13 ounces in the CTR configuration.
• The RFLX suppressor is 10.1 inches long and 1.75 inches in diameter, wrapping 3.09 inches back over the barrel.
• The suppressor adds only 6.9 inches to the muzzle, which keeps overall length down.
• Maxim lists 129 dB subsonic and 134 dB supersonic at the left ear, the maker’s own measurements.
• The trigger is an ALG Combat trigger, a dependable single-stage unit.
• The handguard is the M-LOK compatible SDX two-piece with a full-length 1913 Picatinny rail, finished in black Type III hard coat anodize.

Frequently Asked Questions

How is the CTR version different from the CQB version? Maxim builds the SDX RFLX SBR as a CQB model with its Gen:7 CQB stock and a carbine model with a Magpul CTR stock. This part number is the Black carbine version with the Magpul CTR.

Do I need ATF approval to buy this? No. As of October 9, 2026, ATF no longer requires Form 4 approval, registration or a tax stamp for short-barreled rifles. It transfers on a Form 4473 with a background check through a Class 3 (SOT) dealer. State and local laws still apply. Taking an SBR across state lines still requires an approved ATF Form 5320.20.

Is the RFLX suppressor rated for full-auto? Maxim lists the RFLX suppressor’s firing schedule as full auto rated. It is made from titanium and 17-4 stainless steel with a DLC and Cerakote finish.

Buy the Maxim Defense SDX RFLX SBR .300 Blackout 8.5-Inch, Black, Magpul CTR Stock

As of October 9, 2026, this short-barreled rifle transfers on a Form 4473 with a background check at your Class 3 (SOT) dealer, with no Form 4 or tax stamp; state and local laws still apply. Order through Maxim Defense‘s silencers and suppressed firearms lineup and our Lubbock, Texas team will handle the transfer. The Magpul CTR gives it a familiar carbine feel on a rifle that is anything but standard.