The ones that are built right are not mufflers, they are "folded" tuned pipes [with an inner chamber] that give a true supercharging effect.
The SPAD website........Spads to the Bone.......IIRC is where I found a great step by step MCP construction article. I built a couple of them for OS LA .40s and got a 1500 rpm increase over the stock muffler. The pipe allows the LA .40 to be 3D capable with a 4 pound plane.
Pure magic.