Last reviewed: 2026-04-18 by Jake Terry, FFL07/SOT3, with hands-on install experience on AR/AK/Glock platforms.
The MP5 binary trigger from Franklin Armory — the BFSIII HK-C1 — is a drop-in trigger assembly designed for HK 91, HK 93, MP5, and their clones. Binary triggers are semi-automatic devices: each discrete trigger function fires one round. In Position 3, you fire one round when you pull the trigger and a second round when you release it, delivering two controlled shots per cycle without converting the firearm to automatic fire. This mechanism cuts split times in competition strings and gives you tighter shot groups during close-quarters drills. The selector offers three positions — Safe (Position 1), Semi (Position 2), and Binary (Position 3) — so you retain full manual control over rate and mode.
How It Works
Franklin Armory machines the BFSIII with a pull weight of approximately 4.5 pounds across all three positions. The ambidextrous safety selector is indexed for right- and left-handed shooters, and the housing footprint accommodates most two- and three-position HK trigger packs, including the compact SP5K housing. The patented design lets you abort the release round mid-cycle: hold the trigger to the rear and flip the selector out of Binary mode, and the sear will not drop on release. This abort feature gives you tactical flexibility when a second round is no longer needed.
The Numbers
| Spec | Value |
|---|---|
| Type | Binary (3-position) |
| Platform | HK 91, 93, MP5 and clones |
| Pull weight | ≈4.5 lb |
| Safety selector | Ambidextrous |
| Housing compatibility | Most 2- and 3-position, SP5K |
| Drop-in | Yes (fitting may be required) |
| Compliance | Semi-automatic (ATF), state restrictions apply |
| Manufacturer | Franklin Armory |
The kit does not include the trigger housing, ejector, ejector pin, or ejector spring — you reuse your factory components or source them separately. Some housings require minor fitting; Franklin Armory recommends installation by a professional gunsmith or a Franklin Armory Certified Armorer to ensure correct sear engagement and safe function. After installation, seek qualified training on binary-trigger manipulation before live fire. Browse more binary trigger options for other platforms.
Best Use Cases
Competition shooters use the BFSIII to halve split times in action-shooting stages that demand controlled pairs. Tactical trainers adopt it for close-quarters drills where rapid follow-up shots on steel improve muscle memory without recoil stacking from full-auto. Recreational shooters enjoy the higher rate of fire on range day while retaining the manual trigger discipline of semi-automatic operation. The abort function is critical in all three contexts: if your sight picture breaks between shots, you can stop the release round and reacquire before the next trigger cycle.
Installation Notes
Read Franklin Armory’s installation manual before disassembly. You will need to remove the factory trigger pack from the lower receiver, swap the trigger module, and reinstall the pack. Function-check the selector in all three positions with snap caps before loading live ammunition. Verify that Position 1 (Safe) blocks the hammer, Position 2 (Semi) fires once per pull, and Position 3 (Binary) fires on pull and release. If the sear does not reset cleanly or the hammer follows the bolt, cease use and consult a gunsmith. Never alter any component of the BFSIII — modification voids the warranty and may cause unintentional discharge.
Safe Handling & Compliance
Per current ATF guidance, binary triggers are classified as semi-automatic trigger assemblies — each round fired requires a discrete trigger function, and the device does not convert a firearm to automatic fire. Regulatory status has changed before and may change again; verify current federal and state law before ordering.
- Follow the four rules of firearms safety: treat every firearm as loaded, never point at anything you are not willing to destroy, keep your finger off the trigger until sights are on target, and be certain of your target and what lies beyond it. Review the NSSF firearms safety rules.
- Binary triggers are restricted in California, Connecticut, Delaware, District of Columbia, Florida, Hawaii, Iowa, Illinois, Massachusetts, Maryland, Minnesota, New Jersey, New York, Rhode Island, and Washington. Verify state and local statutes before purchase; we do not ship to restricted jurisdictions.
- Improper installation or use may lead to unintentional discharge. Seek professional installation and qualified training before live fire. No refunds are issued after installation has been attempted.
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