How do I add to a rig with bones?

This picture shows an arms rig on the right which has bones in it. I want to put the gun on the left into the arms rig on the right, but I don’t know how to do that because there’s bones in the arms. Is there some way I can Motor6D the gun to one of the bones?

I ended up just attaching a Motor6D from the HumanoidRootPart to the gun’s handle, and having the gun move separately from the hand. Anyways, the gun isn’t showing up in Moon Animator, but it’s showing up in the roblox editor. Does anyone have any idea why that’s happening?