The Sig Sauer M400 is an AR-style semi-automatic rifle chambered in 5.56 NATO with a 16-inch barrel. It ships with a 30-round magazine and a 6-position Magpul SL-K stock. The Tread V2 variant includes an M-LOK handguard and a stainless steel barrel with the ROMEO5 Gen II compact red-dot sight included. 
 

Key Specifications 

• Brand: Sig Sauer 
• Model: M400 
• Caliber: 5.56 NATO 
• Action: Semi Auto 
• Barrel Length: 16 in 
• Color/Finish: Black 
   

Detailed Features 

• 16-inch stainless steel barrel 
• M-LOK handguard for accessory mounting 
• 6-position Magpul SL-K collapsible stock 
• ROMEO5 Gen II compact sight included with Tread V2 package 
• 30-round magazine included

Owners of the M400 consistently point to the Tread V2 package as strong value, noting that getting a ROMEO5 red-dot bundled in keeps the total cost lower than buying components separately. The M-LOK handguard and Magpul stock combination draws praise for being light and well-fitted out of the box.

Ergonomics get positive marks overall, though a minority of owners say the Magpul SL-K stock feels a bit stiff until it breaks in. The 16-inch barrel length is seen as a practical compromise for home use and range sessions without being unwieldy.

Reliability with standard 5.56 NATO ammunition is generally reported as solid, with typical AR-platform feeding behavior. Some buyers note the trigger is acceptable but is often the first upgrade they make, which is a common sentiment across this price tier.