An ultrasonic cleaner with either TCE or MEK.

You might try a lightly Flitz-treated mop chucked in a drill. You know that solvent works. Or simply a bore brush.

I was cleaning some 17 Ackley Hornet cases with flitz and got some in a few of the necks. I think I got it all with a q-tip. I want to be sure. I do not like to tumble these cases because of the size. I do not want any media stcuk inside, the results would be explosive. Any idea how to get the Flitz out of the case. I was thinking boiling water. SOme rubbing alcohol maybe.