Sorry but I have to call BS on some of this stuff about the strength of the No4.
Back in the 1970's the father of a shooting mate had been involved at Lithgow in the testing of No4 actions after conversion to 7.62 NATO. Many rifles passed and went on to serve as full bore target rifles. However quite a significant number failed and became scrap metal. Anyone with experience in shooting 308 Win cartridges in the converted No4s will have found that even though tested, cases still stretched from firing. Bolt thrust is the product of pressure times the area on which it acts ie the internal diameter of the case head. At least the 7.62 NATO is within cooee of the .303 BRIT. Going up in case size to a magnum is just crazy (because bolt thrust varies with the area of the case head means it increases with the square of the radius) You might get away with it for a Youtube video but sooner or later someones going to wear it.
In respect to the No1Mk3*, in Australia a great many were sporterised to a variety of wildcats and semi-official wildcats. In the old days the Victoria Police ballistics section had an entire wall devoted to photos of those rifles after they had blown up, together with photos of what was left of the head of firer.