I'm with Jerry45 on this, as if it shot well beforehand, and went to pitts after putting it up for a while, it shouldn't have changed, but shot the same afterwards too. The only thing that could cause this, that I could think of, is a dirty barrel, and the time allowed the carbon/copper to settle in and dry. Also, that action shouldn't move around, at all, so you need that looked at.
Try plugging the muzzle with a patch, then fill the barrel with a good cleaner, and let it sit for a couple or three of days, then, after draining it, clean it as normal, using a brush, then a patch with a jag. I'd try this before purchasing a new barrel just yet.