(You need OpenIV in order to access the game files)

Everything except the drum mag goes here:

C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\mods\update\x64\dlcpacks\patchday8ng\dlc.rpf\x64\models\cdimages\weapons.rpf\

the drum mag goes here:

C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\mods\update\x64\dlcpacks\mpexecutive\dlc.rpf\x64\models\cdimages\weapons.rpf\

also if you don't want that goldish tone on the m203 you will have to edit a line in weaponcomponents.meta located in:

C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\mods\update\update.rpf\common\data\ai

once you right click and edit the file (OpenIV latest update includes a text editor) search for "COMPONENT_CARBINERIFLE_VARMOD_LUXE" (line 1245)
then look for "<TintIndexOverride value="2"/>" (line 1259) and change the value of '2' to '0'