BATTER UP !! Put Me In Coach
Here we are in Grey Matter. No, not the runny grey stuff in yer head. That Grey area where the "Law" is ambiguous.
In Grosest Brittania, should you exit the Pub with Car Keys in hand, it shows "intent" and one is subject to immediate arrest. Same Same walking down the street at night with Crow Bar in hand. Intent. That does not apply here. At least not in any court.
Performing a Cartridge Conversion involving a loading slot thru the Recoil Shield is "Manufacturing a Firearm Without License." Legal so long as you do it for yourself. If you try to sell it you become an Unlicensed Firearms Dealer and the BATFE will take great interest in you. That is one Bear you DO NOT want to poke. BATFE take no prisoners and have NO sense of humor what-so-ever. Now, possessing that shoulder stock is not criminal. Attaching that shoulder stock is.
Doing a conversion with a "Drop in" does NOT manufacture a Firearm. A percussion gun, only becomes a firearm with the cylinder installed. DO NOT attach the stock with the cartridge cylinder installed. BATFE again.
Now the problem. You cannot and will not get a definitive answer to an ambiguous question from BATFE. If you ask the same question of 3 BATFE agents, you WILL get at least FOUR different answers, driven by interpretation. However, if only ONE of them determines you have committed and offense, it's Bracelets time. Remember, BATFE has NO sense of humor. It was surgically removed at birth. DO NOT poke the bear.
There can be varying opine on parts of this topic. But an opinion is like certain body parts. Here is the question, do YOU want to spend every dime you can beg, borrow, or steal, defending yourself in court?? DO NOT poke this Bear. YOU WILL LOSE!!
STAY SAFE OUT THERE