The reasons for reduction in accuracy can be very simple. First off due to your experience with rifles I am sure it is not a matter of technique. I am guessing you're an above average shot.

I would look at such things as; Have I changed my powder, sabots, lube, cleaning procedures, cleaning chemicals, even loading techniques. If all of them are the same as when you were getting exceptional accuracy, then I would start checking simple things like the scope or sights. Are the screws all tight? Does anything look out of place? I had a rifle about sent me over the edge one afternoon. We are talking a tack driver. I finally frustrated beyond belief looked and noticed the screw was almost worked all the way out of the left to right adjustment.

If your sure everything is the same and you have changed nothing, then I would suggest checking the barrel closer. Get some J.B. Bore Paste and scrub the barrel. I do my barrels every five range sessions whether they need it or not. The only difference is I run only one patch of it.

If that does not fix the problem, then I would lapp the barrel. There are two ways to lapp a barrel. You can fire lapp them or old fashion scrub lapp them. I scrub lapp almost every rifle I own at some time in its life. Sometimes more then once. This will usually make the most stuborn rifle behave and come back around.

Hope you figure out the problem. Yes rifle barrels will wear out but we are talking thousands and thousands of shots. In fact most report that the accuracy gets better with age because you are wearing all the ruff places out of the barrel which is the same thing that lapping does.

To check and see if your barrel needs lapping. Put two cotton balls on your patch worm and run that down the barrel slow, and back up. After that drop a bore light or in the case of a in line look into a bright light through it. Can you see any of the fibers from the cotton ball? If you can then you need to lapp the barrel. The amount of cotton it is catching is directly related to the amount you have to lapp.

Good Luck...