Yugoslav M59/66s main difference is a 22mm rifle grenade launcher integrated with the top of the barrel. As such, they can launch NATO standard (22 mm) rifle grenades with special blank ammo. When doing so, shooter is provided with a additional extra thickness rubber butt pad as seen here (http://ecx.images-amazon.com/images/I/51vfmnL1CwL.jpg).
The butt pad can be a lootable item that makes the recoil a tiniest bit more controllable, if you decide to make it.