Simply running water though is not going to do it well enough. You need a cleaning jag and patches. If you have no idea what powder was used, try cleaning with water-wet patches, rubbing alcohol/water patches, and smokeless solvent (Hoppes) patches and see which seems to work best.

If that crap has been in there awhile you are certain to have corrosion. I'd never buy a used ML without dropping a bore light down it first.