I'd like to suggest ARMS as a common word - you don't (however much you may be against the trade itself) hear of an ARM trader...
Perhaps there is such a thing as 'an arm (and a leg) dealer' - somebody who trades in used prosthetic devices!
However, I think it's the other kind of 'arms' - the things at the side of your body with hands at the end of them - that will prevent this word ever being accepted in Chi.
There is also the third person singular of the verb 'to arm'. (He/She arms the troops, the doors on the aircraft etc)
I don't know if 'arm' (= 'weapon') is ever used as a singular noun except in compounds such as 'firearm' or 'sidearm'.
