This is the design I came up with for a specialty gun for my dnd gunslinger Hackel. It's a .50 caliber precision musket with a scope mounted above an .80 caliber wall rifle. Both have had their breeches modified to accept reloadable chambers halfway between a metallic cartridge and a breech-loaded swivel gun's swappable chamber. Each barrel has a manual breech block to keep the cartridge in as well as a dual extractor system, allowing the handle to be pulled back along the stock rail in order to extract one or both shells depending on the positioning of a latch. All the mechanics down to the springs and pins are modeled and the wall thicknesses are calculated to the best of my understanding of materials and limited gunsmithing knowledge. I tried to model the materials as accurately as possible and the total weight came out to around 20lbs, so it's a chonky rifle.

Modeled in fusion 360