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303-25 been around for a while. I would imagine there is one somewhere here in NZ.
Try Hugh Bradley at Stager sports. His barrels are pretty well priced and he's been in the game for bit so would be...

My only thought on this is that if yor are paying for a brand new barrel why pick the 303? The same money will get you the 25 cal that he has already keeping it original if in spirit only.
Main...

Yup really need a P17 bolt to swap if you use a 3006 family head size. This can be done on a p14 so its not out of the question and cheaper than buying another rifle/action so this is still something...