Mid 20th-21st century. The AR-15 Sporter-1 (SP1) is a semi-automatic variant of the AR-15 series of assault rifles. It was produced by Colt, the same manufacturer of the military M16 on the same production lines as the military rifle.Huey » 27 Jun 2022, 2:58 pm » wrote: ↑ That the Armalite Rifle 15 that became the Colt M16 are not the same weapon as the COLT AR 15 SP1.
That is why there are two different lowers. If there was ONE design you would not need two different lowers.