Here's my question: would it be possible to drop a parts assembly (like a pistol lower with all the trigger/sear parts installed) in the ultra sonic cleaner for a bit to get the carbon buildup out of that parts assembly without having to disassemble and clean each piece individually?
YES it will....
I was a watchmaker/gunsmith.
Watchmakers routinely clean tiny assemblies in ultrasonic cleaners to avoid having to disassemble them and risk lost or damaged parts.
It's also faster then full disassembly.
That's why ultrasonic cleaners are so good..... they can get into tiny areas no other cleaning method can reach and clean them ultra-clean.
Compared to a ladies watch movement the smallest pistol assembly is gigantic.
As long as you give the cleaner enough time to work, warm the solution, and rinse the items properly they'll come out virtually surgically clean.
The trick is making sure you get a light coat of lubricant into all areas that are subject to rust.
Lubes like CLP Breakfree will "creep" into all areas eventually, but you need to make sure.
I've used a few drops of CLP on a toothbrush to "scrub" clean parts to leave a thin coat of lube to prevent rust.
For a firearms assembly you can't or don't want to disassemble, I've used an air brush to spray a fine mist of CLP into the assembly.
WARNING.... DO NOT inhale the CLP fumes....you will NOT like it.
Pointers....
Warm solution cleans faster-better. Either let the unit run until the solution warms up, or warm the cleaning solution by sitting the container in hot water, or if you're using a water-cleaner mix, just use hot water.
Ultrasonics will remove painted sight and safety markings.
Water based cleaners will take longer, solvent based may remove them instantly.
KEEP YOUR FINGERS OUT OF THE TANK WHEN IT'S RUNNING.
Bone marrow and ultrasonics don't play well together. This is not an instant ouch, it takes a some continued dipping your fingers in. Just don't.
If you use a solvent based cleaner like cheap paint thinner you really won't need to use a rinse until the solvent gets really dirty. Until then a rinse is really wasted time. Also you don't need to run the rinse in the ultrasonics.
Simply submerge the parts in a pan full of solvent.
If you're using a water based cleaner, just submerge in a sink full of hot water to rinse.
Be careful what cleaners you use.
As example standard Simply Green and other harsh liquid soap cleaners like the Purple cleaners and Greased Lightning WILL damage aluminum and WILL damage anodized color finishes and will do it faster in an ultrasonic tank.
Use a basket to keep parts off the bottom of the tank. Ultrasonics work better if parts are in a basket or hung by wires.
NEVER run the tank even a few seconds with no liquid in the tank, they burn out.
No, don't put your contact lenses in the tank. They're destroyed instantly.
If you think the tank isn't working hold a strip of aluminum foil in the solution. The sound waves will blow tiny holes in the strip.