Currently, Merkel has two frame sizes for the double rifles, M140 are built on a 20ga size frame, the M141 is built on a 28ga size frame. Merkel 28ga and 410's are both built on a 28ga size frame. I have handled a number of postwar Merkel rifles and shotguns, my observation is that the frame on rifles is different from the shotgun frames.
The 12 and 20 are different sized frames, the 16 is a good question. Next to me I have a postwar Sauer 16 and a postwar Merkel 12, the 16 "looks" smaller than the 12 and does weigh about 6oz less. If I can find some time I will take some measurements.
