I would imagine your average Fedex employee wouldn't know what a firing pin is or what it looked like. I would just take the slide off the frame and that would probably suffice. Revolvers would be tougher, most probably could not do this, particularly with a hammer mounted pin.
It has been my experience that some, maybe most, Fedex don't know their own rules when it comes to shipping handguns. The last 2 times I shipped a pistol, I disclosed to them them that it was a pistol as you are required to do but I did not ask to have it shippped priority overnight (by 10:30 AM delivery, the most expensive way). I waited to see if they would ask me how I wanted it shipped and they did. I asked what my options were and I chose 2nd day which was considerably cheaper.