Crest 235 Ultrasonic cleaning solution in a heated ultrasonic cleaner.
Plastic parts shouldn't be a problem, so long as the fluid isn't overly hot. It will, however, likely remove any paint markings.
Soft metals, like brass, aluminum, and copper might be pitted by the ultrasonic action.
I wouldn't just dump the entire trigger assembly, with plastic, into the sonic cleaner, simply because getting the water between the metal and plastic out afterwards would be difficult.