The CMMG DISSENT Mk4 SBR .300 Blackout is a factory short-barreled rifle that packs .30-caliber power into an ultra-compact, bufferless package (brand part no. 30A080F-AB). CMMG’s patent-pending Compact Action houses the entire recoil system in the upper receiver, so there is no buffer tube, and a side-folding stock with adjustable length of pull and cheek riser mounts directly to the rear. The 6.5-inch 4140CM barrel has a 1:7 twist and a ZEROED linear compensator on a 5/8×24 thread, with an adjustable .750-inch gas block for tuning to supersonic or subsonic loads. A forward non-reciprocating charging handle, AR-15 style controls and a 5.5-inch M-LOK handguard with a top Picatinny rail make it easy to run. Two 30-round Magpul .300 BLK PMAGs ship with it.

Key Specifications

• Chambering is .300 Blackout on CMMG’s Compact Action direct impingement system.
• Barrel length is 6.5 inches in 4140CM salt-bath nitride, rifled at a 1:7 twist.
• The muzzle device is a CMMG ZEROED linear compensator on a 5/8×24 thread.
• The gas block is an adjustable .750-inch unit for supersonic or subsonic ammunition.
• The handguard is a CMMG 5.5-inch M-LOK design with a top Picatinny rail.
• The stock is a side-folding stock with adjustable length of pull and an adjustable cheek riser.
• Weight is 5.6 pounds, with a length of 16.5 to 26 inches.

Frequently Asked Questions

Why choose .300 Blackout in this platform? 300 BLK has loadings for many uses, including suppressed fire, and the adjustable gas block lets you tune the DISSENT for the supersonic or subsonic ammunition you run.

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.

What is the Compact Action? It is CMMG’s patent-pending design that houses the entire recoil system in the upper receiver, which removes the buffer tube and lets the stock fold to the side.

Buy the CMMG DISSENT Mk4 SBR .300 Blackout, 6.5-Inch, Armor Black

CMMG built the DISSENT to put .30-caliber power into the smallest practical rifle footprint. 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 CMMG‘s semi-auto rifles lineup and our Lubbock, Texas team will handle the transfer, and it ships fast to your Class 3 dealer.