AS Designs ARHK: Drop-In Forced Reset Trigger for MP5 & G3

The ARHK is a self-contained forced reset cassette that drops into a stock HK roller-delayed housing with no lower swap. Most of the MP5 forced reset field asks more of your gun: complete AR-fire-control-group lowers that replace the factory lower, or AR-FCG builds fed by a trip kit. The one existing drop-in cassette, the Rare Breed FRT-RD3, is limited to the 9mm MP5 and SP5. The ARHK installs directly into the factory OEM polymer trigger housing with no modifications, no external slip trip, and no buffers, and it does it across the whole roller-delayed family rather than a single 9mm host. You keep your original lower and drop the cassette in.

AS Designs announced full production on the ARHK after completing torture testing across the MP5, MP5K, G3/91, HK11, HK21, HK23, HK33, HK51, HK53, and HK93 platforms. The company reports every tested host functioned correctly and held up under abuse. The cassette is built on the same ARC-Fire technology that powers the AS Designs forced reset selector, so the mechanism is a known quantity moved into a new form factor rather than a ground-up experiment.

The ARHK uses a three-position selector: Safe, a true disengaged Semi, and Active Reset. That middle position is the practical differentiator. In Semi, the ARHK behaves like a normal single-stage trigger with no forced reset in play, so the gun feels stock for range work, transport, or anyone handing it to a new shooter. Flip to the third position and the ARC-Fire clutch engages, driving the forced reset where the bolt carrier's cycle mechanically resets the trigger after each shot and holds it until the action returns to battery. One press still fires one round; the mechanism just resets the trigger faster than your finger can.

Reset feel is where AS Designs claims a real advantage. The reset is gradual rather than abrupt, which matters most on the .308 hosts where a harsh reset can beat up your trigger finger. Early hands-on reports from the HK community describe the ARHK reset as smooth even in a G3, a platform that punishes a poorly tuned forced reset. The cassette is aligned with Navy-style housing markings, with SEF support to follow, and is ambidextrous. It works with all bolt carrier types, so you are not matching the cassette to a specific carrier profile. For the broader mechanics of how forced reset triggers operate and their legal footing, see our MP5 forced reset trigger guide.

The ARHK is designed to span the HK roller-delayed lineup rather than a single host. Compatibility covers MP5, MP5K, AP53, G3/91, HK21, HK31, HK11, HK32, and HK33 clones and variants, across both 9mm and .308. It comes with your choice of a 9mm or .308 ejector to match the host you are running, so the same cassette design works whether you are on a 9mm MP5 or a .308 G3.

The one hard exclusion is CETME. AS Designs will not clear the ARHK for CETME platforms and cites safety concerns; despite the shared roller-delayed heritage, the CETME fire control geometry differs enough that the company keeps it off the list. If you run a .308 battle rifle and want forced reset, the ARHK targets the G3/91 pattern specifically. For a broader look at running an FRT in a large-frame .308, see our AR-10 forced reset trigger build guide, and for the ARC-Fire selector this cassette is built on, read our AS Designs ARC-Fire review.

The existing MP5 forced reset field splits into three approaches. Complete lowers from Mars ship a finished AR-FCG lower you pin in place; turnkey, but a per-host purchase and a full lower swap. Trip-kit systems from Trinity and the AS Designs Super Safety lower are cheaper to enter but require an AR-FCG lower you build or supply, plus assembly and timing. Drop-in cassettes install into the stock factory housing with no lower swap and no build. The ARHK is not the first cassette; the Rare Breed FRT-RD3 already works that way, so the comparison that matters most is against it.

The ARHK wins that comparison on price and breadth. It runs $549.99 against the FRT-RD3's $615 to $620, and it covers the whole roller-delayed family, MP5 through the .308 G3/91 and the HK-numbered rifles, where the Rare Breed cassette is limited to the 9mm MP5, MP5K, SP5, and SP5K. It also sidesteps the FRT-RD3's fitment split, where the AMBI version only functions in pictogram-marked frames and the SEF version only in letter-marked frames, and the ARHK is built on the ARC-Fire mechanism developed specifically for roller-delayed guns. The trade is timing: the FRT-RD3, Mars, and Trinity units ship today, while the ARHK is a pre-order for early August 2026. Browse the full trigger catalog and compare specs side by side in our component catalog.

The ARHK treats the HK roller-delayed platform the way the AR world has had it for years: a drop-in part, not a lower rebuild. Keeping the factory housing, covering both 9mm and .308 hosts with a matched ejector, and running a true disengaged semi set it apart from the complete-lower and AR-FCG conversions, and the ARC-Fire foundation means the mechanism is not unproven. At $549.99 it prices just under a complete Mars lower while asking far less of your gun.

The only catch is the calendar. This is a pre-order with a first 1,500-unit drop set for early August 2026, so buyers who need forced reset in an MP5 today should look at the in-stock complete lowers and trip kits instead. For everyone building around the stock housing or running more than one roller-delayed host, the ARHK is worth the wait. Compare it against the current field in our best MP5 forced reset triggers guide.