Reloder 22, like many powders, can vary some from lot to lot. Are you used the same batch?
Also, if you found "light rust" in the chamber, there was probably light rust in the bore--which can indeed change pressure....
I have read that before, and saw a little difference on some other powders. Normally I'm not up near max, so I check a group out of a new jug and keep rolling.
Unfortunately, I have no clue on the RL22. I bought 7, 1# jugs all on the same day. I think 5 from 1 store, 2 from another. They have a different label style so those are easy to separate.
But the other 5... I've moved housed and things happen.
I'll go ahead and say. I used to run 52gr, and brass only lasted 4 loadings. So probably hotter than it needed to be.
It is currently blowing primers out of the head at 49gr.
The gunsmith had the barrel off, and says it is clean with his borescope.
I've not used this guy before. I wish I could send it to Lee, but last we talked, he was retired.