I'm going to re-barrel my Ross Mk III

[Sunray]

"...have read they are 12TPI..." That doesn't mean much. There are Withworth threads(square) and American that are not the same thing. I'm not sure which the Ross used, but it matters. Rumour has it that at one point they used Withworth and left hand threads but switched to American at some point.
So does your smithy having the proper tools to remove and install a Ross barrel. Mostly the correct bushing and, more important, the correct action wrench. A pipe wrench or adjustable wrench is not the correct tool.
There are some guys here who discussed it in 2012/13.
https://www.milsurps.com/showthread.php?t=41835

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[tiriaq]

"...have read they are 12TPI..." That doesn't mean much. There are Withworth threads(square) and American that are not the same thing. I'm not sure which the Ross used, but it matters. Rumour has it that at one point they used Withworth and left hand threads but switched to American at some point.
So does your smithy having the proper tools to remove and install a Ross barrel. Mostly the correct bushing and, more important, the correct action wrench. A pipe wrench or adjustable wrench is not the correct tool.
There are some guys here who discussed it in 2012/13.
https://www.milsurps.com/showthread.php?t=41835

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.

[browningautorifle]

to see if that's "available" in .303 brit.

Now I get it, he meant if the LINER was available in the caliber desired. Had to read it again.

If anyone is interested, I could take measurements of the shank.

I too have line drawing of said barrel shanks...could post them.