I use a large size (10 gallon) ultrasonic cleaner daily on a large volume of modern firearms and have learned a few things-
Pre clean your weapon. An ultrasonic is great but it is not a cure all. You have to remove as much loose crud beforehand. I use a water based cleaner similar to Simple Green- field strip and spray the item down and let it soak for a few. I will usually use a cheap paint brush to give a bit of a scrub and soak again.
Rinse with water and into the US.
The hotter your cleaning solution the better, I run mine at 150-170F (This is dependent on the finish of the firearm however- see below)
It typically will not remove hard baked carbon, copper or lead deposits.
They can and will mar certain finishes, faux casehardening, applied finishes i.e. CeraKote, DuraCoat and such, lower class anodizing and some blue finishes.
If you are going to clean such finishes use a cold solution, lighter soap ratio and shorter time in the machine
I will not put finely fitted assemblies into an ultrasonic, it can cause fretting of the finished surfaces (don't believe me? Throw a precision bearing in there, it cost me a set of expensive skateboard bearings for my kid
)
It will strip every bit of oil from your item. Be sure to reoil immediately & thoroughly.