The 80% Arms GST-9 Lower Receiver is a billet lower machined from 7075-T6 aluminum alloy. It is supplied in an 80% finished state, requiring final machining by the end user. The lower is treated with a Type III hard coat anodizing for corrosion resistance.
The GST-9 features an integrated magazine flare for improved reloading and an ambidextrous-ready design. Compatible with standard AR-15 upper receivers and most mil-spec parts kits. Requires a jig and tooling to complete the fire control pocket and selector holes.
This lower receiver ships directly to the customer with no FFL transfer required. It is sold as a non-firearm in compliance with ATF regulations regarding 80% receivers. The GST-9 is designed for experienced builders familiar with ATF guidelines.
Jake Morrison’s shop notes
GST-9 lower has a reinforced magwell area, 0.125" thicker than standard lowers. Noted slight overhang on jig plate fitment – needs deburring on rear trigger pocket edges. Anodizing is consistent at 0.0012" thickness, confirmed with micrometer.
Tested: 4651 rounds bench-cycled. Pocket tolerance measured: within 0.001″ on this batch. Weight (actual): 26.6 oz on shop scale. Units shipped from Phoenix in January 2026: 116.
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