A few years ago, discussing the armed FD crew concept with a senior UAL Captain, I asked him...
Suppose, that during the flight, you had a violent disagreement with
your First Officer (and, make no mistake, during the flight, that F/O
does belong to the Commander, like it or not, by the younger crowd

) and his reply was...'I'd shoot the SOB.'
Hmmm, seems to me a pretty good reason to keep firearms firmly off the FD.