Why do they use a black oxide finish on their barrels and not a mang phos park finish?  

After shooting my rifle last night I put it back in the gun case along with an FAL I have biult(and parked myself).  I guess some moisture had gotten in the case (humidity) and when I opened the case at home this morning I was horrified to see large orange spots on the barrel and front sight of my AR15 and not a spot on my FAL.  I have scrubbed the rust off and the gun looks fine again but I am seriously thinking about parking all the steel parts on the gun that have the black oxide finish.  From my point of view the black oxide finish sucks!

FYI, I don't normally leave my rifles in a gun case overnight but I was at my family's farm when I went shooting and stayed overnight before comming home. Lots of little kids so guns stayed locked in the case.