Sig Sauer M400 SDI XSERIES, Semi-automatic Rifle, AR, 223 Remington/556NATO, 16 Carbon Steel Barrel, Anodized Finish, Black, Magpul SL Stock, SIG Flatblade Match Trigger Low Profile Gas Block With Mid-Length System, 30 Rounds, 1 Magazine RM400-SDI-16B-P The M400 SDI X Series is the out of the box performance solution. The X Series features a 16" chrome molly barrel with a free-floating M-LOK handguard, SIG Flatblade Match Trigger, ambidextrous bolt catch and release, Magpul SL stock, and mid-length gas system.