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The gun needs a carrying case with spots for each of the different barrels. If your intention if to cast guns to sell, you could design inserts for a stock briefcase and cast them in foam epoxy for reduced weight.
Old dueling pistol cases all seem to have wooden dividers covered in velvet:If you did them as an epoxy resin kit, the kit builder could cover the epoxy pieces with velvet and mount them in a box. Maybe there is a stock hardwood box available; a lot of cheap hardwood stuff was coming out of China a while back.