Method one is the use of an ultrasonic cleaner. Method two is use of a tumbler. Both methods require one preliminary step -- removal of the spent primer. Neither step should be tried without removed of the spent primer.
Universal de-capping dies that fit the standard thread of reloading presses provide a convenient way to remove the spent primer. The only contact with the casing is at the prong that removes the spent primer which saves premature wear on the die surfaces.
As a trial, I dropped 50 9mm cases into my Lyman Ultrasonic cleaner after removing spent primers. During the de-capping process, noticeable grit was transferred to my hands from handling the cases. The Ultrasonic cleaning did a creditable job of removing excess powder residue and other grit. However, there were still burn marks and tarnish leaving the clean cases dis-colored.
As I wanted a cleaner look to the finished reloaded ammo, I ran the cases through a Lyman Turbo Pro 1200 tumbler with medium crushed walnut media. A 15 minute cycle of vibration in the crushed walnut media resulted in cases that had a relative clean and polished look.
A close inspection of cases before and after the cleaning process revealed that the Ultrasonic cleaner did remove the excess powder residue and grit; but, the cases were left with powder burns and a tarnished look. Clean, but tarnished.
The tumbling cycle did remove the powder burn and tarnish from the cases. However, the cases were coated with residual dust from the crushed walnut media. A short cycle through the Ultrasonic cleaner did remove the residual media dust coating.
Interestingly, nickel-coated cases clean nice with a shine using Ultrasonic only. Brass cases need the tumbling action to attain a polished case surface.
It is a matter of personal choice -- Ultrasonic, tumbler or both. For me, the decision was both -- for the additional cleaning utility of the Ultrasonic cleaner. With a change of cleaning solution, you can field strip pistols and run them through the Ultrasonic cleaner. Or, clean jewelry or most other metals.
The Ultrasonic Cleaner is multi-function while the tumbler is just a single function case cleaner. Both do a good job of cleaning with a slight edge to the tumbler for a more complete cleaning of the case interior and primer pocket.
The downside to the tumbler is the residual media dust left on the cases. However, a short blast of compressed air should remove that dust. And, there is still a case cleaning step prior to resizing and trimming and final reloading.
Images below show the cases before cleaning, after ultrasonic cleaning and after tumbling cycle.