I've recently been very impressed with the Foaming Bore Cleaner from Outers. A can is just $6.88 at Walmart but it cleans the copper fouling out easier and faster than all the big name brand copper cleaners I've purchased over the years. Easier than scrubbing with JB's paste, also.
BTW, I don't think a gun is ever too clean, it just hasn't been shot enough to be fouled properly. /ubbthreads/images/graemlins/smile.gif