I have a M4 rifle.  Is it safe to store the rifle upside down?  As in muzzle pointing to the floor.  I'm doing it because I want to let the oil and other stuff run down instead of staying inside the receiver.

Should be fine. Don't sweat it.

Yes, it is safe, and not a bad idea.  I've read it as a recommended way to store rifles before.

Been storing mine that way for 10 years.

No big problem with upside down storage except it's easy to knock over your rifle.  While not likely with an M4, you certainly don’t want to ding the muzzle.

Brownells sells a cartridge shape gizmo with an O-ring that you place in the chamber when storing your rifle upright.  Supposedly it stops oil from running down the barrel and into your receiver.  

When using this thing, I’d think you would want to check for any pooled oil in the chamber before firing the rifle.  You’d also want to remember to remove it.