my first thought was copper too and it's also not unheard for something have come loose
i'd start there before changing anything with your load

and even if your cleaning regularly unless you're specifically going after copper with something like sweets
it can build up to a point of hurting your accuracy

my hero Rex explains this very well in his sniper 101 videos this one Part 40 about coppering and powder fouling
https://www.youtube.com/watch?v=_KRU...BZi0vDCIcEPxUn

this vid too
https://www.youtube.com/watch?v=Gcmu...EPxUn&index=45



this might help