Parkerizing is a build of around .003" per side, and the parts are produced semi undersized for these additional build ups.
When you start spray coating on top of the parkerizing, and the parts not also further undersized to allow for the coating build up that can be .010" or more per side on top of the .003" of parkerizing, then it's tthe tolerance stacking are blow to hell (on top of say a key that way over sized produced in a raw state to begin with) .
As for solutions, one is to get such parts that are produced correctly undersized to begin for the additional build up plating or coating over parkerizing surfaces, or in the way back machine such as the first DPMS chrome plated carrier where the key where just too large/tight, lap the side and bottom of the carrier key to remove the additional coatings in the bearing areas, leaving you with a key back to width and high in spec instead.
Also to note, when checking a carrier key against a upper, start off by checking with the charging hand removed, then add the charging handle into play once you have confirmed that is not the bearing side of the key just above the carrier that are the problem first.