To install with OpenIV you replace the provided w_ar_musket_ files in these two places: 

GtaV\mods\x64w.rpf\dlcpacks\mpindependence\dlc.rpf\x64\models\cdimages\weapons.rpf
GtaV\mods\update\x64\dlcpacks\patchday8ng\dlc.rpf\x64\models\cdimages\weapons.rpf

And you must replace the provided .meta files in these two places as well:

GtaV\mods\update\update.rpf\dlc_patch\mpindependence\common\data\ai
GtaV\mods\x64w.rpf\dlcpacks\mpindependence\dlc.rpf\common\data\ai


If you have any problems with the gun not showing then replace the gun and attachement files in the other patch days as well.

Thanks for downloading.