OK lads, I made a couple of mistakes. It is Azaaton, and just to be sure I made a reverse check and entered Azaaton in the online "RomajiDesu". It produced the text as on the rifle.
But I still find it totally implausible that this is a Japanesetransliteration of a ship's name. That is, for me, just wishful thinking. The signature of the engraver seems more likely, as suggested in previous posts. Written in Katagana because Hiragana characters would be very awkward to chisel into the metal in the confined space.
And it certainly is a piece of work worth signing.