I have put a lot of thought into that topic and have an idea. I will provide the drawing tommorow if the thread is stil up. The point was to replicate 12ga dimension case using precussion caps as a primers (they are unrestricted where i live). So the idea is to use a milled round body as a main part of a casing on one end it has a hole for a precussion cap and a three threaded holes on a radius, halfway in between the edge of the body and the center cap hole. What holds the cap in place and serves as the rim of the cartidge is the washer that has three threaded holes corresponding to the position of the holes in the main body. The washer also has a smaller center hole for a firing pin. Now whats holding the washer to the body are just three stubby bolts with the internal hex seat that thread into the washer and the main body and sit flush within the washer and do not pertrude into the bolt of the gun. Alright now the main problem of this "design" is that the lenght of the thread on the washer is very short so im not sure if the small bolts will resist the shear force of extraction for example in a pump action. Experimentation is needed to test that. Pic related is the type of bolt that connect washer and main body