The mod replaces the assault SMG in the game with the MTAR-21.
Installation: Upload files to Grand Theft Auto V/update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/models/cdimages/weapons.rpf
Skins can be replaced by editing the w_sb_assaultsmg.ydr file, renaming the desired skin in the folder as mtar21_d and replacing the existing one with it.