I personally wouldn't hesitate cleaning any metal item in an ultrasonic cleaner, as long as it is dried and lubricated afterwards. I used to work as a commercial dive technician offshore and would use the ultrasonic cleaner to clean all the divers diving equipment, SCUBA - rubbers, plastics, steel, and brass for 1st and second stage regulators, hoses, o-rings, springs, pins, valves etc... You name it.
The whole point of the US cleaner it to get to those places you can't see or reach. Mind you the media we used was couple hundred dollars per liter, BIOX from Divex, safe for breathing apparatus. Also, some vinegar based solutions can lift the chrome right off of brass!
I'd happily US clean ANYTHING, even my rifle bolts, as long as you dry it properly (compressed air - in a can if you don't have a water separator on your compressor). But, if you have your doubts, don't do it. No problems.
Here's an article that sheds a little light on US cleaning for SCUBA kit. If it can be used for breathing apparatus, it can be used for rifle parts. Hell, I'd even hazard a guess that instrument technicians probably use ultrasonic cleaners in their Oxygen safe/clean environments.
http://www.gmcscuba.com/pdf/ultrasonic% ... tfalls.pdf