Bob,
The most likely explanation for speed variation is not the barrel, it is the Varget. My last batch of Varget requires a full grain more powder to maintain the same speeds as the previous batch - in the same barrel. I have never had a BR get noticably faster with more use. This is a known phenomenon and a source of much frustration when having to switch to a new batch. The most consistent powder I have ever used was RL15, but it is hard to find in quantity.