This happened occasionally with the No8 rifles too and it was found that it was the brass that was the problem as opposed to the charge. Once the batch of ammo had been used or withdrawn, it was back to normal. I think this thread was aired here some time ago and one of the metallurgists on the forum (JM springs to mind.......) suggested that the extractor could scratch the thin brass case as it clipped over it and it was this that caused the weak brass to weaken further.

Try a different HV ammo brand and see if it does the same. I'm not an ammo person but....... anyway, the original thread went on and on for a bit!