I will add my $.02 worth.

What we teach in Armorer courses is that guns need lubed to reduce friction and protect from rust.  So anything that is steel needs a lubricant with a rust inhibitor.  A generic way to approach looking for friction, is to look at a firearm that has some rounds through it, and you can often times see where the metal has rubbed metal, and that would be a good indicator of where it needs lubricant to reduce friction.  

On the AR15/M16/AR308 charging handle, most are made of aluminum, which means rust is not an issue, but on a used charging handle you often times see friction rubbing on the sides and top along its length, which is where it needs a light coat of lubrication for daily use, and heavier when doing training where you will be doing lots malfunction clearance drills and reloads.  

On the AR15 / M16 / AR308 Bolt carrier assembly, everything is made of steel, which means it could rust, so it needs a rust inhibitor.  If you look at the bolt carrier as if you are behind the gun shooting it, so that the carrier key (gas key) is at 12:00, you will see that the carrier has raised rails at 2:00, 4:00, 8:00, and 10:00, and this is what the carrier rides on inside the upper receiver.  There is a large flat on the bottom of the bolt carrier at 6:00, which is where the carrier rubs the face of the hammer is the bolt carrier moves rearward when cocking the hammer.  If you insert just the bolt carrier (No bolt, cam pin, firing pin or retaining pin) into the upper receiver by itself, you will see that it freely moves back and forth inside the upper receiver, and actually is loose enough that it rattles slightly if you add a little slight rotation both directions as you push/pull the carrier in/out of the upper receiver, this shows that the carrier actually free floats in a sense, so it only needs a very minimal amount of lubrication as there isn't a whole lot of friction.  

Now assemble the entire bolt carrier assembly, and slowly insert it into the upper receiver, doing the same slight rotation test as mentioned above, as you insert it into the upper receiver, and you will see that it has the same free float looseness is it travels into the upper receiver, right up to the last about 3/8" of forward travel, which is where you get a large amount of friction as the bolt rotates and locks into battery with the barrel extension lugs.  As you slowly pull the bolt carrier assembly rearward, you will also see that the first 3/8" of rearward movement you will have lots of friction, which is caused by the bolt carriers rearward movement that pulls the bolt rearward causing the bolt to rotate clockwise during its unlocking from the barrel extension lugs.  

Then if you look at a bolt that has quite a few rounds through it, you will see rub marks on the front of the bolt lugs, and rubs marks on the back and sides of the bolt lugs.  The rub marks on the front of the bolt lugs happens during the locking cycle, where the bolt lugs have traveled through the barrel extension lugs, and the front of the bolt lugs then engage the threaded pinned barrel, at which time the bolt gets pushed reward into the moving forward bolt carrier, and the cam pin rotates to the left side in the bolt carrier cam track, and the bolt then rubs against the back end of the barrel, this is the friction that happens during the locking into battery cycle of operation of the firearm.

The rubbing on the back and sides of the bolt lugs happens during the unlocking cycle.  As the bolt carrier is pushed rearward from gas pressure on a gas gun, or a piston rod pushing it rearward on a piston gun, the bolt carrier as it moves rearward will pull the bolt rearward away from the barrel extension lugs.  When this happens, the cam pin will rotate to the right in the cam track of the bolt carrier, which causes the bolt to rotate clockwise as its pulled rearward away from the barrel extension lugs, this is where the most friction happens, and where the lubrication is needed to make the firearm run reliably.

So taking that into account, the entire bolt carrier needs a light lubrication on all parts so it is glistening like a glazed donut for rust protection.  For friction I would suggest putting a drop or two on the rails of the bolt carrier, and then the 7 bolt lugs.  What we teach in Armorer courses is that each bolt lug needs its own drop of lube for daily carry, so 7 lugs = 7 drops, and if you are going to purposely shoot lots of rounds for training then soak each lug so it is dripping as you cannot get them wet enough.  Think of lubrication like a boat trailer, if you are going to drag that boat trailer 5 hours across a state to go to a particular lake, then you should probably lubricate the hubs before you go, the same thinking I like to apply to a firearm where I know that I am purposely going to shoot 500 or more rounds through on a range session then I will heavily lubricate the friction areas.

As for what lubrication to use, that is personal preference, but I would recommend using something that is made to use on firearms, and be very careful of using things that are not made for firearm usage so you don't cause damage to things like metal finishes and treatments.  My personal preference is Slip2000 "EWL" Extreme Weapons Lube, as it doesn't burn off when doing high round count shooting which is great as it reduces friction so we don't see malfunctions happening due to friction, and at the end of the training session the gun is dirty but the "EWL" hasn't burnt off so the fouling is kind of like soot which makes cleaning very easy.  It also works well in extreme cold weather.


CY6
Greg Sullivan "Sully"
SLR15 Rifles
DefensiveEdge.com
(763) 712-0123