Grease will work.
As for the muzzle device use a crush washer.
I don't use thread locker because it is not needed.
You can if you want to.
Wait before you do that. I got a Smith Vortex flash hider and it did not come with a crush washer. I called to complain since I really wanted to shoot that day. Smith said that flash hider (same as used by the SEALs) does not utilize a crush washer and told me to put the flash hider on and tighten to specs. Find out from the manufacturer before automatically putting a crush washer on.
