Procurement will use the NSN when something else would serve the same purpose with more efficiency.

Some day there will be groups of experts scattered around the world that will discuss the AR-15 and all its variations just like the nihonto-a-philes do now with Japanese swords.  They'll look at scraps of handguards trying to figure out the lot number, and about once a year a lower receiver engraved with Mattel will emerge.  All of the discussions in their internet forums will use part numbers and NSN's to describe the parts, instead of plain language descriptions.  Eventually there will be so many marked Mattel the experts will be brought to their knees and forced to admit they don't know what is going on.  And when someone comes up with an unknown SP-1, OMG!, don't install a new barrel, bolt, trigger, or safety unless you have been blessed as an expert and have access to correct parts.  If you paint your old nasty AR, you're going straight to hell, too.