The Steyr STM-556 is a rare piston-driven AR-15 variant that attempted to integrate features from the Steyr AUG, such as a quick-change barrel system. Introduced in 2011 and released commercially in 2020, only 300 units were produced for the civilian market. While featuring an adjustable gas system and an ambidextrous bolt catch, its proprietary barrel system, unconventional ergonomics, and high cost contributed to its commercial failure. The rifle demonstrated reliable function during testing but was ultimately deemed a miss by reviewers.
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