In 1994, a man named Mark Westrom, owner of Eagle Arms, purchased the husk of the Armalite corporation, and acquired its trademarks. Westrom wanted to create a new commercial .308 AR pattern rifle, and did so under the Armalite AR-10 name. He developed an AR-10 which borrowed some elements from the AR-15, and introduced it in 1996 with pretty reasonable success. In addition to versions with Picatinny rails and AR-15 style charging handles, he also had a retro version with the top-mounted charging handle so iconic form the original AR-10s.
One substantial change to the pattern was the use of modified M14 magazines in the new Armalite AR-10B rifles. This was done because of the Assault Weapon Ban that had gone into effect in 1994, which prohibited manufacture of magazines holding more than 10 rounds. There were no cheap .308 AR magazines available at that time (no MagPul yet…), and modifying ubiquitous M14 magazines was the best option available – so that’s what was done.