My good friend and hunting bud had a 25 built and was trying out different bullets. He took the time to run close to a hundred rounds through the new barrel to "break it in" something he is not usually accustomed to doing.
Like you it shot the Hornady standard and Light Mag loads into little clusters, but just about everything else we tried wouldn't drop below 1.5".
He worked up to my load of RL-22 with the Partition and it shot VERY well, but still not into the ragged holes of the factory stuff. We also found that both the Nosler and the Bergers did foul his barrel within am matter of around 25 shots, to the point the groups would start to open up even bigger.
That may be the whole issue with yours. It may be building up copper and throwing your groups. I know for a fact the jackets on them are much softer than what Hornady or most anyone else other than probably Nosler uses.
With mine, I simply just have so many rounds down the tube it is WELL polished. I usually only clean it about once a year. She still shoots well enough that I don't mind what little build up we get form the Nosler's we run through it. Then again nowadays we probably don't run 20 rounds a year through it either compared to years ago when I was playing with every new load I could find for it.
About the only bit of advice I could offer would be to possibly run some JB through it on a tight patch and polish it up a bit. I wouldn't get overly aggressive, but I have found with a couple of rifles that a half dozen patches or so, slathered down with JB, and gently stroked through the barrel several times each will polish it up something good.
