HK MR556 and HK416 Forced Reset: MR3 vs ARC-Fire

Factory semi-auto MR carrier, buy the FRT-MR3. HK416 or BRN-4 carrier, buy the HK ARC-Fire kit. Nothing else in the decision outranks that one check.

A forced reset is timing work between the carrier and the trigger group. In the MR3 the carrier's rearward stroke drives the trigger forward into reset and pins it there until the action returns to battery, so every round still takes its own press. Rare Breed built that timing around the semi-auto MR carrier and rules out the HK416 and BRN-4 full-auto carriers by name. That is a fitment boundary, not a tuning problem. Rare Breed publishes no buffer, spring, or gas workaround for a full-auto carrier, so the rifle either has the right carrier or it takes the other path.

The ARC-Fire earns its price by covering the wider carrier list. AS Designs documents MR556, MR223, HK416, and BRN-4 carriers across the A1 and A4 versions with no work done to the stock carrier, which is the reason to choose it. The trade is that it is a selector system rather than a complete trigger. You supply a fire control group whose trigger carries the selector clearance cut, and you confirm the receiver's detent path before you pick a throw.

Because the parts that make a forced reset work are matched to a carrier, not to a lower that happens to take AR-shaped pins. The MR556 puts the controls where an AR shooter expects them and takes an AR-pattern fire control group, which is exactly why the wrong assumption is easy to make. The carrier is where the resemblance runs out.

Two different mechanisms are on sale here. An FRT is a complete replacement trigger with the reset mechanism living inside the trigger group. A forced-reset selector, the family the Super Safety made familiar, replaces the safety and cams a compatible trigger forward as the carrier cycles. Our forced-reset selector guide works through that distinction across AR-pattern hosts. On an HK it also sets the size of the purchase: one box, or a selector kit plus a trigger machined to clear it.

The ARC-Fire sold for AR-15s and the ARC-Fire sold for HK hosts are two products. HK Parts lists the HK416/MR556 package under its own SKU, HKP-22890-M, with selector levers cut for HK receivers, and says the ordinary AR-15 retail package is not interchangeable with it. Order against the SKU, not the product family name.

HK's own literature keeps these guns apart and so should the parts list. The MR556 is the U.S. commercial semi-auto rifle derived from HK416 architecture, which is a lineage statement rather than a parts-interchange statement. The FRT-MR3 follows the commercial line precisely: MR223 and MR556 on the factory semi-auto carrier, with both A1 and A4 selector drums in the box so either selector geometry is covered.

Barrel length does not split the trigger family. HK's current MR A4 line lists an 11-inch MR556 A4 SBR and a 16.5-inch MR556 A4 rifle as two configurations of the same short-stroke-piston, rotating-bolt semi-auto rifle, so a trigger decision made on one carries to the other. The operating balance does not carry. The shorter gun runs a different gas impulse, and a setting proven on the 16.5-inch rifle is a guess on the SBR until you shoot it.

HK416 generations need the same restraint. HK states that HK416 A5 trigger components and pins are not compatible with the standard HK416. HK Parts lists A1, A3, and A5 coverage for the HK ARC-Fire kit, which is a claim about that kit and not a claim that HK416 fire-control parts cross between generations. Buy against your configuration, not against the generation list.

Everything an MR223 or MR556 install needs arrives in one box. The $525 FRT-MR3 is a single-stage, three-position unit, Safe to Semi to FRT, and it ships with anti-walk pins and screws, both A1 and A4 selector drums, selector lever roll pins, and two T10 Torx wrenches. Nothing gets sourced separately and nothing gets machined.

The selector is the detail that shows it was designed for this rifle rather than adapted to it. It is ambidextrous and carries a 45-degree position matching the MR platform's own selector markings, so the controls still read correctly to the shooter after the swap. The assembled unit runs 5 oz in 7075 anodized aluminum with heat-treated 4140 and S7 steel internals, and the curved and flat blades cost the same, which makes blade shape a preference instead of a budget line.

The carrier restriction is absolute. If an HK416 or BRN-4 full-auto carrier is already in the gun, either put the factory MR semi-auto carrier back in or move to the ARC-Fire path. Our FRT-compatible guns guide runs the same host-first check across other platforms.

The $249.95 HK416/MR556 ARC-Fire V2 kit is the pick when the carrier sits outside the MR3's list, or when you want a two-stage break for ordinary semi-auto fire from a trigger whose geometry suits the kit and carries the selector clearance cut. AS Designs documents the mechanism on MR556, MR223, HK416, and BRN-4 carriers in the A1 and A4 versions with no modification to the stock carrier, and HK Parts lists MR556 A1/A4 and HK416 A1/A3/A5 hosts for the kit itself.

Selector throw is a receiver measurement, not a preference. The kit is sold in 0-45-90, 0-45-180, and 0-90-180, and HK Parts warns that receiver detent geometry on A5 and A1 guns can block the final 180-degree position. Look at the detent path in your own receiver before ordering. When it does not support the long sweep, take a 90-degree endpoint and lose nothing but the throw.

The kit does not include a trigger, and the trigger is not optional. AS Designs specifies either a mil-spec AR fire control group running its pre-cut trigger, or a Geissele MR556 trigger that has received the selector clearance cut. Add that second purchase before you compare the kit's price against the MR3's, because the honest comparison is a system against a system.

The $470 AS Designs pre-cut Geissele is the premium version of that second purchase: the Geissele HK MR556 two-stage with the clearance already machined in. It breaks at 2.75 to 3.0 lb on the first stage and 1.5 to 1.75 lb on the second for a 4.25 to 4.75 lb total, on an M4 curved bow. Its full-size hammer means the separate lever blocker is not needed, which is one fewer part to fit in the lower. Two limits ride along with it. It does nothing on its own without the ARC-Fire kit, and it does not fit the MR762.

Cutting a Geissele for selector clearance voids Geissele's warranty. What the pre-cut version buys is the machining itself, done by the shop that designed the selector rather than at your own bench.

Do not treat the 1.8 mm gas vent HK Parts publishes as an install step. It is a tested starting point for particular A4 and A5 configurations, and the number travels badly. Barrel length, ammunition, whether a suppressor is on the gun, and the gas hardware already installed all change how much carrier energy reaches the reset mechanism.

AS Designs ran the MR556 suppressed without changing the vent, which is the counterexample that settles it: the 1.8 mm recommendation is not universal. Establish your host, carrier, barrel, and suppressed or unsuppressed setup, then shoot the rifle and let it tell you whether anything needs opening up. Drilling is not reversible.

For vocabulary on what you are actually seeing when it misbehaves, the AR-15 FRT tuning guide separates failure to reset from light strikes and hammer follow. Its buffer prescriptions are AR-specific and do not transfer to a short-stroke piston gun. The SIG MCX forced-reset guide is the closest parallel case, another piston platform where the carrier interface decided the parts list.

They are built for a different action. The MP5, MP5K, SP5, and SP5K are roller-delayed guns whose trigger pack sits in a self-contained grip frame, and the forced-reset products sold for that family are cut for that housing. The MR556 and HK416 are rotating-bolt, short-stroke-piston rifles with a conventional lower receiver and a different carrier relationship entirely.

A shared HK rollmark is not a fitment claim, and the 7.62 guns are their own question again: AS Designs publishes a separate install path for the MR762. Start from the host and the carrier, then work outward to a specific part number. Our forced-reset trigger buyer's guide sorts the category by action family, which keeps the roller-delayed products on their own side of the line.

Eligible rifle FRTs rest on the 2025 DOJ settlement, in which the district court concluded that they cannot be classified as machineguns. ATF describes the settlement as covering eligible FRTs and states that prohibitions under state law remain independent of it. The MR556 and HK416 are rifles, so that federal framing applies to them directly.

State law is where this is actually decided, and 16 jurisdictions currently restrict or ban forced-reset triggers. That matrix tracks each one against its statute. A maker's shipping policy is a business decision and follows the statute only loosely in either direction, so read the law for your own state and check for a local rule stacked on top of it.