Heavy duty airframe grease that meets milspec MIL-G-21164D, this is the thread lubricant that is called out in US military maintenance manuals when installing the barrel nut and lower receiver extension castle nut onto M16 rifles and M4 carbines.
The product is Shell Lubricants’ AeroShell 64 repackaged into 5ml syringes, each containing 3ml of grease. The syringes are tightly capped, and have a plunger stop installed so that messy accidents are very unlikely. Each is also labelled clearly as AR-15 Barrel Nut Grease, and also with an AeroShell 64 batch number. The amount in each syringe should be enough to get even the sloppiest user through 2-3 rebarrel jobs. Used properly it will last at least twice that many.
Ships for free if ordered with any other products.