I believe there is a 10 bore round action listed in the daybook.

This is not the first .500 I have seen, and a friend of mine once had a Black Powder .577.

Remember, Dickson's made round actions for 15 years before nitro propellant came into general use. By their design, there is more material in the action juncture of a round action than a bar action sidelock or a boxlock. Even with ejectors, there is only a small hole lengthwise on each side.

BTW The same gentleman had a .500 BPE Dickson Round Action of about the weight of the rifle in the auction, and the recoil was so brutal he sold it.