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Walmart carb cleaner is cheap and works great.
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Every so often I use it to clean the "deep" crud out of the upper and the lower receivers. I do hold the upper by the barrel, receiver down and spray till the crud stops flowing. For the lower I hold it by the stock with the receiver down and again, spray till the crud stops flowing.
Let them dry (just takes a few minutes) and spray some light lube in to get oil back on the metal parts that will rust. Then I use the Mobil 1 for lubing the parts that need lube.
Brake cleaner works, too.
Be careful, don't get it in your eyes and spray it outside. Carb. cleaner in your eyes is something you don't want to do.