Install: GTAIV\pc\models\cdimages\weapons.img\
In order to get the weapon to function as it should alas one shot before reloading and no shell ejecting, youll need to edit the weaponinfo.xml and replace a few lines there, instructions are in the readme file.
model used: https://3dwarehouse.sketchup.com/model/741815d3-5f6d-4c4c-b6b3-f8db1c41778e/JACO-WESTERN-PISTOL?hl=en