Spengler, as a felon, was forbidden firearms ownership, and could not legally purchase a firearm.
The purchaser of the rifle he used, Dawn Nguyen, committed multiple federal and state crimes in her straw-man purchase and conveyance of the firearm to Spengler, and has been charged.
Weird, criminals don't follow laws... Who would have guessed?
BTW, I am impressed above that you somehow deduced the manufacturer of the firearm used from the initial news reports, given that there are hundreds of manufacturers of AR-15 rifles here in the USA, where the AR has been the top-selling platform for years now. I spent some time working with Col. Dr. Martin Fackler at the Army's Wound Ballistics Lab at the Letterman Institute, and it was often quite difficult to determine the projectile used to produce a given wound, much less what firearm launched the projectile, and that was with the evidence sitting on the table right in front of you. My hat is off to your psychic powers!