The gun shoots arrows, first right click cocks and second fires.
First, git clone https://github.com/trmaa/minecraft_revolver_mod.git && cd minecraft_revolver_mod/.
Then do cp -r ./* ~/.minecraft/resourcepacks/revolver/ and cp -r ./* ~/.minecraft/saves/<your_world>/datapacks/revolver/.
If you shit yourself in the pants while are a masochist, therfore: use windows to play minecraft (who knows why), .minecraft/ directory (folder for turds) might be at C:\User\<PieceOfShit>\AppData\Roaming\... you paste the files of the repo at... I do not want to help you... Go away, please!