I have a couple rifles that take 2-5 shots to settle in after a cleaning and de coppering, one I sold took about 20 rounds. Just a regular cleaning with a carbon solvent, 5-10 patches, not so much of an issue. But nothing 24" off, more like 3-4" at 200 yards for 1-3 shots.
I have several rifles up into the 250-300 round count, no cleaning, with no accuracy loss at all, no hi volume hot barrel shooting however. All they may get is 1-2 patches with light oil before being put away, if even that. You'll find some interesting round counts without cleaning on some of the sniper/military sites.
One theory is some copper is needed in some barrels, to fill in flaws? No idea. What I do know is every rifle is different, and improper cleaning can do major damage, especially to the crown.
I have moved towards only cleaning hunting rifles if accuracy is beginning to go, and often give in after 100 rounds or so and do a light cleaning with a good carbon solvent. But on 3, I have not done any deep cleaning/copper removal for 250-300 rounds. I had one at the range this Sat. With it's favored load it would not shoot better than 4" at 200, when it was always 2 - 2 1/2" rifle at 200. So it will get a full cleaning before this weekend and back to the range it will go. I may just clean all 3 and start over, I don't know. This is the stuff that can drive a guy crazy...
I did have Barnes bullet accuracy issues pre grooved bullets. Since the grooves, they have been as dependably accurate a hunting bullet as any other for me. So if you duplicate the cleaning & accuracy issues, you may just have a gun that really likes to be dirty with barnes bullets. Were I you, I'd shoot it enough, clean and dirty, to make sure you have ID'd the issue. If that pattern is repeatable, alls well, if not, I might look for other gremlins. But even if the pattern repeats, it may make sense to lengthen the cleaning bullet count so long as accuracy holds.