bigjedd,
Response was more aimed towards 450 Ackley and the gunsmith that built the BLR's. If he used locally produced barrels, I'd suggest slugging the one that showed pressure signs as it wouldn't be the first local barrel to be grossly under sized.

In your case, I'd be interested to see whether you still have a problem now that you've cleaned the rifle and put some rounds through it. It'd be interesting to keep a log book and just keep shooting to see whether the issue arises at a similar round count as before.

Maybe chronograph the ammunition as well ... about a decade ago I had some Rem 45/70 loads that gave me a shot string that averaged at 1630fps when it should have been 1310fps.

Weird stuff happens...
Cheers...
Con