It comes down to personal preferences. I myself use my brand new '54 No4 Mk2 more often than I do my 30-06. I do so because it shoots better. With my 174 grain loads, there is not any North American game I cannot take with it. It's my preference, choice and belief that it is the best for me and my hunting preferences. Same as the OP, if the OP chooses to use a 303 then that is his choice.
Far be it for anyone to criticize someone else for matters of preference. The question was not "which is better". The question was "can I get a 303 barrel". More to the point, you were absolutely off the point of this whole thread. In relation to the original question, who cares what you can "fathom", because it has nothing to do with the intent of this thread.