Allen, The 20 ga, would work, but the 28 ga action would be better for a 243 flanged. Merkel's smaller cartridges are now made on the 28 ga size action,in the model 141.

You are correct, the 9.3X74R shortened and formed in a 243 die would do the trick. Though it would be a little loose in the head area of the sizeing die, for re-sizeing, it would likely work with a 243 win die set, and a 9.3X74R shell holder! With 100 gr bullets, pushed at around 2800 fps it would make a cracker-jack double rifle for deer hunting, in the woods, or cross canyon, with a good light weight 2-7X32mm scope in QD mounts!

I've been hunting with double rifles, and cape guns for years, in North America, for everything from tit-mouse to moose, and I have always thought a fine well ballanced S/S double rifle chambered for a flanged 243 Win is a spot the makers, mistakenly, left out!