This is a pretty easy thing to do but not many know of the correct way of doing it
so here is the video

https://www.youtube.com/watch?v=hJ6bo_1BXoM

you guys can ask for any help you need.

also about the sniper rifle in the video, the process of making it was already posted here
https://sherrygames.com/making-of-m83-sniper-rifle/