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...
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.