280 Ross barrel

[Bluenoser]

A groove diameter of 0.31" sounds like the bore could be .303 Brit - possibly done due to the lack of suitable diameter bullets for a .280 Ross. I think you should do a chamber cast and a cast of the rifling with Cerrosafe. I wonder if it could be a .303/280 Ross wildcat, but have never heard of one. The markings suggest it is not a Mk. III barrel rechambered to .280 Ross.