Copy "musou_isshin"Folder to "Grand Theft Auto V\mods\update\x64\dlcpacks\"

Go to Grand Theft Auto V\mods\update\update.rpf\common\data\dlclist.xml
Then use text editor open it, add new line

<Item>dlcpacks:/musou_isshin/</Item>

Save the dlclist.xml file.

Load Weapon: WEAPON_GEN5