[b]*****FOUND IT!!*****[/b]
Taken from page 3 of "The Complete AR-15/M16 Sourcebook" By Duncan Long:
[b]"It should be noted that several Armalite firearms were being developed in differant projects at the same time rather than one after another, as might be suggested by the numbered designations given to the various weapons.
Rather, the number designations were given to the rifles as they were perfected and offered for licensing. Consequently, at any one time several firearms were in various stages of development. Thus, the Armalite designation numbers of AR-1, AR-10 and so forth do not necessarily reflect the chronological relationship as far as when a firearm's design and testing might have been initiated. The numbers do show to some extent the order in which the guns were offered to licensing companies, but not when they actually went into production."[/b]
Funny note though: there was AR's 1,3,5,7,9,10,11,12,14,15 and 16
AR's 2,4,6, and 8 never got off the drawing board and AR-13 was a "hypervelocity airccraft gun"
Cool huh?