Is it a new one? If so I would be inclined to give it a thorough (careful) cleaning - mine was covered with a thin coating of brown-coloured preservative (in the bore too).

I find that Eley Sports (green box) and Winchester Subs (hollow point if you can use them) perform identically in mine out to 100 yards. Single hole off a bipod or rest at 50 yards. Whether these are the "best" or not I couldn't say, but I have found these rounds to be more consistent than Eley Club or any of the CCI stuff. Does fitting a moderator make a difference? I use either a SAK or UK Neil (BOSS) moderator.

Mine has had more than 500 through it and I have noticed no change in performance between the first and last shots.

Hope this helps.