What is a Mk 113 MOD 0?

No such nomenclature. You're probably thinking of the Mk 13 Mod 0, which is a 40mm grenade launcher designed by Fabrique Nationale for use with the FN SCAR. The launcher itself is marketed as the GL1, and was designed for use with the FN-2000. Mk 13 Mod 0 is a USSOCOM nomenclature for a variant of the GL1, designed for use with the FN SCAR.