20 minutes in liquid is not enough time to wet the powder even with unsealed ammunition
WWII contact mines were packed with Nitrocellulose, which often got wet due to leakages. The primer set them off anyway, wet or not, when a ship bumped into them. The contact mechanism, if corroded, may fail to fire the primer and trigger a blast, but Nitrocellulose is not water soluble. The nitrated cotton is dissolved in Acetone, or some other suitable solvent, camphor, or some other plasticizer is added to keep it soft after the solvent evaporates, and the mixture is poured into the mine and fills up the cavity. So, even if the mine casing leaks the sea water won't affect it. Nitrocellulose does not need to be confined because it detonates, so even if the shell of the mine rusted almost completely away as long as an active trigger was in mechanical contact it will explode. I suspect that most of you reading this are familiar with ping pong balls, which are (were?) made of nitrocellulose and when lit by a match burn with a quick flash. One could take many of them, grind them to small particles, pack them in a container, and ignite them with a primer. Will they explode or just conflagrate? Depends on the percentage of Nitrocellulose.
The big questions are:
1) "Will the sonic cleaner cause the liquid to seep past the crimping seal of the casing against the lead bullet?" and
2) "If cleaner gets into the casing will it affect the cordite or the primer?"
If the cleaning solvent dissolves the primer material the bullet will not fire. Among priming materials used today is Lead 2,4,6-trinitroresorcinate, usually referred to as lead styphnate. It is soluble in water to less than 1 hundredth of a gram per 100CC of water, which is essentially insoluble, but can be neutralized by Sodium Carbonate, which is caustic. Cordite, usually coated in Graphite, is not water soluble.
http://design.caltech.edu/micropropulsion/msds_w79.pdfPHYSICAL DATA
Appearance: Rectangular plates
Freezing Point: Not Applicable
Boiling Point: Not Applicable
Decomposition Temperature: EXPLODES at 330 Deg.C (626 Deg.F)
Specific Gravity: 3.878
Bulk Density: 1.5 (g/cc)
pH @ 25° C: 6-7 (solution)
Vapor Pressure @ 25° C: Not Applicable
Solubility in Water: < 0.01%
Volatiles, Percent by Volume: Not Applicable
Evaporation Rate: Not Applicable
Vapor Density: Not Applicable
Molecular Weight: 705.53
Odor: None
Coefficient of Oil/Water: No Data
So, the final answer is: unless the casing to bullet seal leaks cleaning fluid, and that cleaning fluid contains Sodium Carbonate, and during the cleaning process the primer never reaches 360C (620F),
the bullet will discharge if used in a gun.