The Masterpiece Arms Defender 30K is a semi-automatic pistol-caliber platform chambered in 9mm, built around a 4.5-inch threaded barrel (1/2x28) with a side-charging handle. It accepts Glock-pattern magazines and ships with a 30-round magazine, and includes a side-folding hinge with a buffer tube adapter for mounting a compatible brace or stock. The trigger system pairs the Defender bolt with a standard AR-15 fire control group. 
 

Key Specifications 

• Brand: Masterpiece Arms 
• Model: Defender 30K 
• Caliber: 9mm 
• Action: Semi-Automatic 
• Capacity: 30+1 
• Barrel Length: 4.5 in 
• Color: Grey/Black 
   

Detailed Features 

• 4.5-inch threaded barrel (1/2x28) accepts standard muzzle devices 
• Side-charging handle keeps the top of the receiver clear 
• Accepts Glock-pattern magazines, ships with a 30-round magazine 
• Side-folding hinge and buffer tube adapter allow a compatible brace or stock to be added

Owners of the Defender 30K describe it as remarkably compact for a pistol-caliber build, and the side-charging handle draws consistent praise for keeping the top of the receiver clear of a reciprocating handle. Some new owners note the side-folding hinge takes practice to lock in place confidently, but most consider it a minor learning curve rather than a flaw.

Because it runs on Glock-pattern magazines, shooters like being able to reuse magazines they already own from other guns in their collection. The steel upper paired with an aluminum lower draws mixed reactions: some like the balance it gives the platform, while others feel the front end is barrel-heavy at first pickup.

Aftermarket support for the platform is a common talking point, with owners appreciating the ability to swap braces, optics, and handguards to fit their preferences. The overall consensus is a dependable, if niche, pistol-caliber build that rewards owners willing to build it out.