The Remington Mark III does indeed show up on the BATFE C+R List published in 2007 at page 38. I have not spent the time to chase down the question as to whether the Mark III or any signal pistol is properly classified by the BATFE as a firearm, but the IRS does not consider signal pistols to be firearms and they are not subject to firearm taxes. At one time it was considered to be an item that required compliance with the National Firearms Act, but that is no longer the case.
On the related question as to whether it can be considered a destructive device take a look at the useful discussion on the US Militaria Forum a while back: http://www.usmilitar...0-pyrotechnics/
The Mark III was not designed or intended to safely shoot factory 10 gauge shotgun cartridges. Anyone doing so runs the risk of serious injury, in my view.
I hope that info helps you.