Click on the link and Subscribe to the mod.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2417971162

Open the in game settings.




Click on the box next to the mod.




Then click Apply




The mod has now been added to the game.

The weapons will now have a percentage chance to spawn in all game modes.

In the Menu select Submarine Editor.




Open a saved Sub by clicking the folder top left.




Then select a Sub.




Once loaded search for the weapon you wish to try in the Filter e.g. RPG




Click where you want it to spawn.




Click on the Play button in the top left




You are now able use the weapon in a test environment