Aeroshell 33-MS grease for use on the barrel nut, receiver extension, and muzzle threads per mil spec assembly recommendations.
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It is called dissimilar corrosion. The steel absorbs electrons from the aluminum forming finger like strands that eventually turn to aluminum oxide stopping the conductivity and moving beside itself. If you can separate the berrel you want to replace the upper anyway due to the loss of material and stability of the remaining aluminum. The corrosion will pit the surface and go through. The worst part is it is happening to the threaded side and the non threaded side of the receiver. You should inspect the area when it is available. Inside and out. Also look at the bearing surface of the upper. The 4 rails. They will pit and deform without attention. If the front is bad I am willing to bet the rails are as well.
Hey guys – just fyi, Shell recently renamed Aeroshell 33ms with Aeroshell 64, check it out http://site.skygeek.com/PDFs/33msto64.pdf
Try using "Mouse Milk" penetrating oil to release the barrel nut from the upper. I had an aluminum hub for a trim wheel (Million dollar flight sim) that fused (galled) itself onto the steel shaft that it spun on. There was nothing that would un-stick this. We had a couple of A&P mechanics look at it too – one guy suggested trying "Mouse Milk" to see if that would help. We squirted it in as best we could given the access to the shaft and waited about a half hour – It released and we were able to remove the hub.
This stuff is magic…
https://www.amazon.com/gp/product/B0049C7170/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
What about using this grease for BCG lubrication? I imagine it would work great.
just bought some, thanks!