Weld arms to head, but keep animations possible?

Is it possible to weld the arms to the head, without losing the ability to play animations with those arms?

I'm trying to make an fps gun so that the arms follow the direction of the mouse / camera angle in first person.

I've seen that this is possible by welding the arms to the head, and then moving the head alongside the angle, but then I can't play any animations with the arms. Namely, if I weld the arms to the head I have to set the Part1 value of the Motor6Ds nil, so animations don't work anymore.

I've also tried using the transform property of Motor6D to make the arms move up and down relative to the camera angle, but then they don't circle around the camera properly: then they circle around the motor6d joints.

Is there any way to avoid this? Or another way to move your arms up and down with the camera angle?

Any help is greatly appreciated.

TL;DR: How do I make the arms move up and down relative to the camera angle?