If the gun will fit in an oven, put it on the top rack and a rimmed cookie sheet to catch the cosmo below - low and slow like BBQ.
You'll have to wipe it off frequently as the cosmo leaches out.
Or if you live somewhere still hot and sunny, leave it outside in a black plastic trash bag in direct sunshine. I generally use this technique in July and August.
Or fill up a tub with boiling hot water and drop the gun in; the cosmo will float up, and you'll royally anger anyone who has to clean the tub...that goes for the oven above too. (and if you try a dishwasher, get the lady of the house some flowers, "just in case.")
Then when you think you've got it all, shoot it 'til the barrel gets hot and watch a whole bunch more ooze out...
Whatever you do, you'll need at least one, if not four, rolls of paper towel.
As for the bluing, a lot of those degreasers will mess up stock finish before the metal finish.