Chambered in 5.56 NATO, the Sig Sauer M400 Forge runs as a semi-automatic AR-15 pattern rifle with a 16-inch cold hammer forged barrel and a mid-length gas system. This FDE configuration ships with a factory-mounted 2 MOA Sig Romeo-MSR Gen II red dot and a 30-round magazine. It pairs a Magpul SL-K adjustable stock with a free-floating M-LOK handguard for a ready-to-run, optic-equipped package. 
 

Key Specifications 

• Brand: Sig Sauer 
• Model: M400 Forge 
• Caliber: 5.56 NATO 
• Action: Semi Auto 
• Capacity: 30-round magazine 
• Barrel Length: 16 inches 
• Weight: 6.8 lbs unloaded 
• Finish: Flat Dark Earth Cerakote 
• Material: Alloy receiver, cold hammer forged steel barrel 
   

Detailed Features 

• The barrel is threaded 1/2x28 with a 1:7 twist rate and wears an A2 birdcage flash hider. 
• A TriggerTech two-stage flat blade trigger breaks at 3.5 pounds. 
• The 15-inch free-floating M-LOK handguard carries a full-length Picatinny top rail, and the upper has an ambidextrous charging handle. 
• The Magpul SL-K stock adjusts across 6 positions and is paired with a black reduced-angle polymer grip. 
• Overall length runs 32.25 inches collapsed to 35.5 inches extended.

Owners of the M400 Forge line tend to like that Sig ships the rifle with an optic already mounted, since it removes a step before heading to the range. The cold hammer forged barrel and mid-length gas system are generally seen as a durable, low-maintenance combination, and the TriggerTech trigger draws praise for breaking cleaner than many stock AR triggers.

Ergonomics get mixed reactions. The Magpul SL-K stock is lighter and slimmer than a mil-spec stock, which some shooters prefer for handling, though a few find the reduced cheek weld less comfortable on longer range days. The M-LOK handguard has enough room for a light or foregrip without feeling bulky.

On value, buyers comparing the Forge to a base-model M400 point out that the factory optic, upgraded trigger, and Magpul furniture add up to real savings over buying those parts separately. The FDE Cerakote finish holds up well to regular handling in most reports, and the AR-15 platform means aftermarket parts and magazines stay easy to find.