With 50 rounds through it without a clean, I would suggest the answer has already been given to you. Had the same problem with a .243 barrel which I was not cleaning properly. (It also had other 'issues').

The only way to check is to clean it and shoot again. Did accuracy fall off as well ? My .243 didn't, it just blew out the head of the case.

Edit: The only other possibility is a head space issue with a different batch of brass/ammo. Was it all the same batch ?

460 Ackley. I was considering a wildcat 9.3mm Ruger but with velocity results like yours I may shelve the idea.