I store my ARs muzzle down, dustcover closed, mags inserted.
I glad to hear I'm not the only one to store my long guns muzzle down. I found that the lube stays put and out of the stock, and keeps dust from entering the bore.
The AR's Achilles heel has been always been reliability problems based on close tolorances and the inferior design of the gas system, blowing gas and carbon fouling into the breech area and gumming up the works. Although the close tolorances wouldn't be a liability by themselves with a different gas system.
This is primarily why I chose the Daewoo K2 over a CAR-15. I've ran several hundred rounds through my K2 without it choking even once. When cleaning time came, there was hardly anything in the breech area or bolt face to clean out, it dosen't have a dust cover.
To be fair though, in the original Stoner design and specifications, the AR was very reliable. Problems arose when the gummint stuck their fingers in the pie and started tweeking things.
The main problem came from changing the original powder in the cartridges to a slower and dirtier burning propellant. This raised the cyclic firing rate way up putting more stress on the gun, and causing more wear and heat. Then of course the additional fouling from inferior powder.
The gummint in it's infinite wisdom
, instead of going back to original powder specs, tweeked even more by adding the forward assist and telling GIs they needed to clean more often. They even changed the powder specs even more and went with a Win. propellant. H335 is now the standard propellant in GI ammo.
My basic training issue M-16 was a Vietnam Era, 3 prong suppresser type, POS. Accurate? Well I qualified expert with it, but it gagged on about every 5th round. Swore I would never own one.
IMHO, if you shoot it, clean it. If it sits more than six months w/o shooting, clean it. If you don't shoot it regularly, inspect it monthly, and clean if needed.
Uhmmm...........and those three holes in the bolt carrier is to bleed off gas once the bolt starts to unlock. But yes you can put oil in them, just make sure the bolt is in it's locked position before you do, otherwise the first round will burn/blow it out.