extra info.
Gordon, I think that a solvent rinse after a water-based cleaner is better than without that rinse, but did wonder if, in theory, high purity alcohol would absorb the water better. Solvents may "displace" water (remember the trick of drying out a damp distributor cap with WD-40?) but may not absorb or dissolve water.
Since you personally have access to a nice solvent tank, just use that and forgo the water-base solution in the ultrasonic until you can do the complete disassembly. Any Remington trigger assembly will tend to retain fluids in (to name a few) recesses like: hammer spring hole (cap has tiny vent hole), carrier spring plunger, safety spring hole. Solvent retained in those areas will not have the same corrosive effect as residual water, and also remember the tight pins still in their holes can have water driven into areas by the force of the ultrasonic cavitation. You can't "uncavitate", since vacuum is maxed out at low level, but pressure can be built to near infinity.
I have been using ultrasonic machines and have sold them for well over 10 years. The time savings and efficiency improvements for various end-user projects have made more than one customer say that they could kick themselves for putting off buying a unit for so long. Remember that the effect of water is mostly against ferrous (iron-containing) metals, but solutions have been known to attack other metals present in typical gun assemblies. Higher temperatures, stronger solutions, and long immersion under power are the most likely ways to damage softer metal parts, like aluminum and zinc (pot-metal)
A neat test to verify that you have efficient cavitation is to put the end of a strip of aluminum foil in the water after it has run for several minutes to "de-gas", and pull it out every 30 seconds or so. The first time, the foil should be no longer shiny, but frosty. Shortly thereafter, pinholes will become apparent. In a few minutes, the end will be disintegrated into tiny flakes. A powerful unit can really do a number on thin aluminum parts, with enough time. Ever seen a prop damaged by cavitation?
Some of the details that I provided earlier were not aimed specifically at just your situation. I sometimes expound as if there is a group present, rather than an individual. Sometimes adding a small detail here and there can give many more the idea that this info. can be for their benefit, also.
As for the 3" barrel, if you changed the ejector in the frame, the proper way from the factory is to change the rivets that stick out slightly on the outboard side of the frame. When they are riveted tight, filing flush along with a polish and blue is the norm. There is a special cutter that is sometimes able to cut away the upset (internal) part of the rivet, so the assembly can be removed and replaced, and hopefully there is sufficient remainder to re-peen the metal. If not, back to the scrimmage and drive out to replace the factory rivets.
The effort expended makes me recommend using it as a non-magnum. There are some very stout 2n3quarters shells out there, or do you guys have armor-plated doves?
When you want a gun to challenge you, think about this. I re-worked a .410 Citori for a shooter that was unsatisfied with the barrels not hitting together where he pointed. I gave the barrels a small hacksaw job and crown, to test fire and verify that the barrels shot straight, and they both just hit a little high. Added some custom choke tubes and lengthened forcing cones, and he said that he was surprised how well he did on the doves. When he went back out with his 12 gauge, he was disappointed, because, in his words, there was no challenge! He killed basically everything that he was shooting at, since the .410 had forced him to be precise. He said the only drawback for a 24" barrelled O/U was the fact that you couldn't go sloppy on your follow-through. If you stopped deliberately swinging, the gun was so light that it didn't have it's own inertial "follow-through", so that was another reason that he liked about the gun. Can't be counting on anything but proper technique. Kind of like having Bruce Lee staring over your shoulder all the time, just waiting to correct you with a thup to the haid.
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Did some tests on wood with half-ounce #9 .410 through an extra full I made. 30 yard board had about 40 pellets, 40 yard board, about 20 pellets.