Ill leave it up to you decide whether this truly is steampunk or not, to me it is. I always like the more refined, Victorian-Noble style of steampunk compared to the gritty Lower-Industrial style. This is the kind of weapon you would find in the possession of a noble class assassin.
Pulling the trigger sets of the steam-activated firing pin. The pin fires forward, striking the blasting cap on the back of the bolt, propelling the bolt down the hyper-rifled barrel. Simple as pie. Bolts are stored in a belt-clip holster with magnetized sides to prevent the bolts from rattling around.
Unfortunately the use of blasting caps can leave residue in the barrel, so an underbarrel cleaning rod has been provided in the style of many colonial period muskets.
I do have to credit a gentleman on Flickr known only as Hstrang for the hose idea.
Made in PMG .7