If your Mauser is a sporter, then ignore the link I sent earlier, it is for military stocked Mausers.
Originally Posted by BoogieMan
The sight on the 303 covers a 8" target, not that I can shoot irons over 50
Switch to a six-oclock hold, there is no reason (unless your eyes are pretty bad) you can't shoot an enfield with Iron sights pretty far out. Even the No5 the sights are graduated to 800 meters.
You would be surprised how well some of the old military rifles can shoot over irons.
It isn't clear from your posts, what type of shooting are you wanting to do at 300-600 yards? Shooting groups, ringing steel, something else?