My gun wont stay in the same spot

i keep watching tutorials about placing a gun object into the game its works great until i play it and when i look up and down my gun moves like up and down to not staying in the postion on the screen where i pplaced it
heres a video please help me: - YouTube

This happened to me too, but i figured it out, if you player’s scale is not 1,1,1, then it will do that.