The Justicar with the Potshot barrel and foregrip looks a bit odd without a shoulder stock to go with it. The model for the stock could be plundered from the Spearfish or the 880 and added to the Potshot attachment's model, spatially placed within the Justicar's pistol grip to mimic a rifle grip and stock. Can't think of a way to get it to look super pretty, especially since there are multiple pistol grip attachments, but it's the form factor that counts.

my expert mockup, using the default mosconi stock

reference photos of other stocked revolvers