Most of my shooting with a wide variety of rifles over the last 5 years has (coincidentally) involved copper or copper-jacketed bullets. The copper fouls the rifling much more quickly than expected, and accuracy degrades. If this is the culprit, it will come right back after cleaning the barrel with a dedicated copper solvent.
An over-heated barrel usually shows vertical stringing of the groups. If your loss of accuracy is more random (or you do the common sense check of letting the barrel cool for 15 minutes), then look for another culprit.
