AS Designs ARHK Drop-In FRT Cassette

The AS Designs ARHK is a self-contained drop-in forced reset cassette for HK roller-delayed guns, and it takes a different path from the rest of the MP5 forced reset field: no lower swap. The Mars unit ships a complete AR-FCG lower, and the Trinity and AS Designs trip-kit systems need an AR fire control group lower you build or buy. The Rare Breed FRT-RD3 is the one existing drop-in cassette, but it is limited to the 9mm MP5, MP5K, SP5, and SP5K. The ARHK installs directly into the factory OEM polymer housing with no modifications, no buffers, no auxiliary parts, and no external slip trip, and it spans the whole roller-delayed family. You keep your stock lower and drop the cassette in.

The cassette is built on the same ARC-Fire technology that runs the AS Designs forced reset selector, and it uses a three-position selector: Safe, a true disengaged Semi with a normal single-stage pull and no forced reset, and Active Reset in the third position where the ARC-Fire clutch engages. That disengaged semi is the practical differentiator over Super-Safety-based systems that keep the forced reset behavior live in the second position. The ARHK is aligned with Navy-style housing markings, with SEF support to follow, and is ambidextrous.

Fitment spans the roller-delayed family rather than a single host. The ARHK covers MP5, MP5K, AP53, G3/91, HK21, HK31, HK11, HK32, and HK33 clones and variants across both 9mm and .308, and you pick the ejector lever to match the host: 9mm, .308, 5.56, or no lever. It works with all bolt carrier types. AS Designs is explicit that the ARHK will not run on CETME platforms for safety reasons. It is not purchasable yet. AS Designs completed torture testing across every listed platform, started part production on 2026-07-27, and said on 2026-09-07 that part production is complete and it is waiting for parts to reach its facility before assembling the 1,500-unit first batch; the exact drop date will be announced once building starts. Verify current federal, state, and local law before purchase; forced reset triggers are banned or restricted in roughly fifteen states.