Hiding a body part when idle

hello, sorry for the delay i was quite busy

so: the weapon already has its own material which i replace with the original one. in unreal engine, the model has several materials and for the weapon one, if i want it to look exactly the same as what it looks like in game without modding, i must use the material the game provides. unfortunatly it doesn’t make it invisible meaning this isn’t how the game normally does it, so i can’t simply make another material as it already uses one :<