As I understand it, the Rolling Block breechblock will loosen up and gain headspace on high pressure loads over time due to not having enough support for the breechblock. They were chambered for 7x57 at one time, and apparently had some troubles with that over time.
The falling blocks like the 1885 and 44-1/2 do not suffer that issue, they will handle it.
I expect that is why the Ubertis are not chambered for high pressure cartridges even though they are modern steels. You may get a better technical answer to your question on the ASSRA forum. Should perhaps look at a book called the Single Shot Breech Loading Rifle, by Ned Roberts and Ken Waters, and Singleshot Rifles and Actions by Frank de Haas.
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