One of my uppers has a bolt carrier group that seems to drag a little too much and fails to completely lockup after inserting a fresh magazine.  I have to use the forward assist to complete lockup.  The carrier doesn't fail too lockup after firing a round.  I've tried 2 different lowers but problem remains.  Anyway, I'm just curious what lube/grease readers like to use on the exterior of the carrier and on the bolt itself.    I generally shoot at an indoor range so I don't consider environmental conditions (sand/dust/dirt) to be an issue.  Thanks guys!

Might check the carrier key and gas tube alignment.

If it chambers fine while shooting, then it should also chamber fine if you lock the bolt back, insert the mag, then push the bolt catch button. If it does, then the most likely explanation is that you're riding the charging handle.