Obviously a proper barrel vise and action wrench are required if a barrel is to be removed and replaced on a Mk. III rifle.
Ross Mk. III barrels can be in tight.
Mk. II (except for the II** rifle) have a unique rapid twist modified buttress left hand thread. There is a set screw which locks the barrel in place, properly indexed. Once the set screw is removed, the barrel will unscrew without difficulty, frequently without the use of a barrel vise or action wrench. The barrels were designed to be changed without difficulty.
The Mk. III is different. Its barrel has conventional right handed square threads. I have a Mk. III barrel in the shop. If anyone is interested, I could take measurements of the shank.