Can anyone confirm whether that attachment beneath the barrel is a laser sight or something else? These days a laser module doesn't have to be nearly so large...

That looks like a grenade attachment to me, as insane as it sounds.

Or maybe a flashlight one?

It's been a while since I saw the series, but I think it was some sort of cross / holy water shooter deal, see the cross-section of its barrel here .

As for it being a laser sight, the series is meant to happen in the 1920's, and while there is anachronistic technology present, there's nothing that would approach lasers so far as I remember.

If i recall correctly, I saw it used as a grappling hook in the first or 2nd ep (when they go onto the ship) and crash into the statue of liberty!

Our anime club at school just finished this series and I can confirm its a grappling hook