The hard to find Shell aerospace grease that meets mil-spec MIL-G-21164D for barrel nut assembly on AR platform upper receiver threads. As used on complete Umbrella gov/military sample builds, offered here in .5oz jars for your own barreling projects. Unless you run your own shop, this jar will likely be the only one you will ever need. If in doubt, buy two and give one to your buddy* when you realize that you'll never use it all.
*Not suitable for personal lubrication.