BH: If you put a stock on that revolver when it is in its cartridge conversion state you have just mfg'd an NFA firearm, ie: pistol with a shoulder stock when that bbl is under 16". Set up as you have it, it is in the same legal status as an unregistered sawed off shotgun or machine gun!
If the gun is set up only as a percussion revolver with that stock then you are ok, you'd also be ok if the gun had the mentioned longer bbl..
By Federal law, the NFA, you have to apply to the ATF by filing the appropriate paperwork, pay a 200.00 tax and it takes quite a period to time for approval.
Generally speaking you've also gotta get your Chief law enforcement officer (or an acceptable substitute) to sign off on the application form or establish a trust. If you don't, what you depict is a felony with a very hefty fine &/or jail time! No joke, serious business.