The folks at Merkel say their big bore rifles are built on the 12 gauge receiver with no additional modifications to the metallurgy. They also said the same is true of Rigby who use the Merkel actions. Merkel's biggest is the .500 but I believe Rigby goes up higher on the same action. I guess it depends on how much more pressure is generated in the chamber of a .600 verses that of the .500. Also, the barrel width will be a factor to consider.