Which forced reset trigger fits an HK MP5, MP5K, SP5 or SP5K, how the ambi and SEF housings differ, and what to check on the gun before you order one.
A forced reset trigger is a semi-automatic trigger. It fires one round for each pull, exactly like the trigger it replaces. The difference is in the reset: instead of waiting for your finger to come forward on its own, the bolt carrier drives the trigger back to the reset point as it cycles. Your finger meets a trigger that is already returning, so the rhythm of a fast string is shorter and more even.
That also means the trigger is tied to the cycling of the host far more closely than a standard fire control group. Ammunition, buffer weight on AR pattern hosts, and the general condition of the gun all show up in how the trigger feels. On a gun that short strokes or runs dirty, a forced reset trigger will feel worse than a good two stage. On a gun that runs, it feels very fast.
The MP5 family does not use an AR fire control group, so an AR forced reset trigger will not go in one. The parts that fit the roller-delayed HK 9mm guns are the RD3 units, built as two stage triggers with a three-position selector:
Ambi or SEF is the choice most people have to make. If you shoot from either shoulder, or you run the gun with a support hand high on the handguard, the ambi housing saves you moving your firing hand. If you shoot right handed and want the classic look and feel, the SEF housing is the one. Nothing else about the trigger changes between them.
If you already own the trigger and something in the housing is worn, the spares are listed separately: the locking bar spring kit, the selector detent kit and the safety detent kit cover nearly everything that goes soft with round count. Owners of PTR pattern clones need the PTR selector kit rather than the standard one.
The MR223 and MR556 are HK rifles, not MP5 pattern, and they take the MR3 units rather than the RD3:
The B&T pistol calibre carbines sit in the same part of the rack as an MP5 without sharing any parts with one. The APC9 and APC45 take the FRT-AP3; the SPC9, SPC45 and GHM series take the FRT-SG3.
If your host is a standard AR-15 rather than an HK, the AR pattern parts are the FRT-15 drop-in assembly, the FRT-15L3 and FRT-15C3 units, and for AK pattern guns the Diablo AK-47 trigger.
On an AR pattern host the trigger is only half the job. A forced reset trigger is driven by the carrier, so carrier mass and buffer weight decide whether the reset is crisp or vague. That is why the counter carries the Odin Works HFRT buffer system, the heavier H3 buffer and full-profile M16 profile carriers alongside the triggers themselves. On the roller-delayed guns there is no buffer to change, which is one of the reasons the RD3 units behave so consistently.
Keep the fire control group clean and lightly lubricated, and look at the trigger the same way you look at any wear part: if the reset starts feeling different, stop and inspect rather than shooting through it. The armorer’s maintenance kit covers the FRT-15 spares; for the HK units, contact us before replacing anything inside the housing.
Everything here is a semi-automatic component. Each round fired requires a separate pull of the trigger, and we do not sell machine guns or machine-gun conversion devices of any kind. The rules that apply to forced reset triggers have changed more than once in recent years and are not the same in every state. Check the current federal position and the law where you live before you order.
No. The MP5 uses a self-contained trigger housing, not an AR fire control group. The parts that fit are the RD3 units.
Where the selector sits. SEF has one on the left side, AMBI has one on each side. The trigger mechanism is the same in both.