Sig Sauer Tread Handguard, 15, Fits Sig Sauer M400, Barrel Nut Not Included, M-LOK, Black HGRD-TRD-E15-MLOK-BLK This 15" Tread M-LOK free float handguard features lightening cuts and is a two screw replacement for the Sig Tread rifle series. While compatible with other AR-15 pattern rifles" it does not include the barrel nut which would be required to install the handguard on a rifle other than the Sig M400 Tread. Barrel nut is sold separately.