How to add physics to first person arm component?

Hi. So, here is my problem. I am making a first person game, and have two components. My first component is the third person character which the player cannot see, and the first person arms which the player can only see. My problem is that, the third person character has collision, but the arms have no collision. I went in and looked to see if the arms had collision attached to it and it does. Any ideas on why its not working? Thanks.