ArmaLite's AR-10 entered the competition late, hurriedly submitting two hand-built "production" AR-10 rifles based on the fourth prototype in the fall of 1956 to the United States Army's Springfield Armory for testing. The AR-10 prototypes (four in all) featured a straight-line stock design, rugged elevated sights, an oversized aluminum flash suppressor and recoil compensator, and an adjustable gas system. In the fourth and final prototype, the upper and lower receiver were hinged with the now-familiar hinge and takedown pins, and the charging handle did not reciprocate and was not attached to the bolt carrier. For a 7.62mm NATO rifle, the AR-10 prototype was incredibly lightweight at only 6.85 lbs. empty. Initial comments by Springfield Armory test staff were favorable, and some testers commented that the AR-10 was the best lightweight automatic rifle ever tested by the Armory.