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Franklin Armory BFSIII HK-C1 Binary Trigger Review

Is Franklin Armory's binary trigger a gimmick or a gotta have it? We put it to the test and find out!

Franklin Armory BFSIII HK-C1 Binary Trigger Review
James Tarr found the Franklin Armory binary trigger for HKs to work exactly as advertised, with binary at near full-auto speeds! But…for a stiff price.

Within the past year Franklin Armory has introduced two new models of their binary trigger, the BFSIII (binary firing system, 3rd generation). One of these is for the CZ Scorpion EVO 9mm carbine/rifle, and the other for HK-pattern long guns/handguns. The BFSIII meant for HKs intrigued me, as the same trigger pack could be used in 7.62x51mm, 5.56x45mm, and 9x19mm rifles/pistols (HK 91/93/MP5). The possibilities made me go hmmm.

A quick primer for those of you unfamiliar with binary triggers and their potential. Like the standard trigger the binary has a Safe and Semi-Automatic mode, but it also has third, binary, mode. In binary mode, pulling the trigger makes the gun go bang, and releasing the trigger makes the gun go bang again. If you can get on and off the trigger quickly a binary trigger allows you to drastically increase your rate of fire.

I own a Zenith 9x19mm Z-5K pistol, which is a clone of the MP5K. It is equipped with an SB Tactical SBT5KA side-folding brace which makes it eminently portable while resembling the orginal MP5K PDW. The MP5 was originally designed as a submachine gun. The question I had was, would the Franklin Armory BFSIII offer a practical increase in both rate of fire and hit probability or would it simply be a range toy? Hmmmmm.

To find out I secured a sample of one of these trigger packs for testing. Officially it is the BFSIII HK-C1 and if you look it up the first thing you’ll note is that it ain’t cheap. Suggested retail on this sucker is $699.99, but it is a fraction of the cost of a registered transferable selective-fire MP5 (they start around $15,000 and go WAY up from there).

I had three questions:

1. Does it work as advertised?

2. What kind of cyclic rate can the average shooter achieve with this product?

3. Does it have real-world utility?

And, finally, 4. Is it worth the money?

What’s In The Box?

The BFSIII HK-C1 arrives in a small box with all sorts of bits and pieces included. In addition to the assembled trigger pack you get a partially disassembled bilateral selector lever, extra hammer spring, two small Allen wrenches, some slave pins if you need help swapping out your hammer spring, and red stickers to place on your receiver underneath your new selector. Considering the reputation Germans have for constructing overly-complicated firearms designs you might be shocked at how simple it is to remove the trigger pack from the lower.

Franklin Armory BFSIII HK-C1
What you get in the box with the Franklin Armory BFSIII HK-C1. The stickers are for the sides of your gun, presumably if your rapid fire draws the attention of local law enforcement.

To install the BFSIII HK-C1 first, remove the lower from the rest of the gun by popping out the pins in front and back. Once the lower receiver has been detached from the rest of the gun pivot the selector lever all the way up to the vertical and the lever will slide right out the side of the receiver. If you happen to have a bilateral safety selector lever, once you pivot it to the vertical your right side pops off and then the left slides out. Once you get the safety selector out of the lower receiver the trigger pack just lifts out in one piece.

Franklin Armory BFSIII HK-C1
Take the lower receiver off your HK by popping out a few pins, rotate your selector switch to the vertical, pull your selector out the side of the receiver, and your factory trigger pack will come right out of the lower receiver.

Franklin Armory provides an extra power hammer spring with their binary trigger pack. If you plan on using the trigger system in a 9mm MP5-style gun you don't need the extra power hammer spring. However, if you plan on installing it in a 5.56mm or 7.62mm gun you will. Replacing the hammer spring is a rather complicated process and I won't even try to describe it here. Instead, track down Franklin Armory’s instructive video on the subject.


An optional right-side safety lever is provided with the binary trigger pack. If you're a right-hander, you probably will not want to install this lever. When pushed all the way down into binary mode it points nearly straight downward and will hit your trigger finger.

Franklin Armory BFSIII HK-C1
If you don’t want to install the right side selector lever in the Franklin Armory BFSIII, you can just attach this button. If you’re right handed the right-side lever pokes your trigger finger in Binary mode.

To use the Franklin Armory trigger pack in your gun you will need to take the ejector, ejector pin, and ejector spring out of your trigger pack and install it into the Franklin unit. I understand why they designed their unit in this way. The 9mm guns have different ejectors than the 5.56mm and 7.62mm guns, however it's still irks me that with a nearly $700 unit I’ve got to finish putting it together.

Franklin Armory BFSIII HK-C1
The factory HK trigger pack on the left, compared to the Franklin Armory BFSIII HK-C1 on the right. Tarr has already moved the ejector, ejector pin, and ejector spring from his HK trigger pack to the Franklin unit.

Swapping out the ejector, ejector pin, and ejector spring is simple and only the matter of a few seconds. The ejector is a big vaguely L-shaped lever that is held at the top left side of the trigger pack by a short fat pin. The ejector at rest really isn't under any tension by the spring in the back, so you can easily push out the retaining pin with any sort of punch. The swap only takes about 15 seconds if you don't have to do any surgery on your ejector to get it to fit into the trigger.

What do I mean by surgery? Apparently the Zenith ejector is a little thick, and they tend to bind in the slot in the Franklin housing. Your ejector should move freely, and not bind at all, anywhere. However if it does, it’s a simple fix. I took a Dremel tool to the back half of my ejector and ground it down a little bit on either side. Not much, just basically enough to take the black finish off, which is all it needed to then pivot smoothly in the slot in the trigger pack.

Franklin Armory BFSIII HK-C1
The Franklin Armory BFSIII with the three parts you’ll need to move over from your factory trigger pack, the ejector, ejector pin, and ejector spring. The Zenith factory ejector is apparently a little thick for the slot in the Franklin trigger pack, and Tarr had to grind it down just a little so it would pivot smoothly (the shiny part on the ejector).

Squeeze, Please

The trigger pull on the Franklin Armory binary pack in semi-auto provided a 6.5-lb pull that was slightly gritty. Even though the length of travel is slightly less than my 5-lb factory trigger pack I couldn't shoot it any faster than the original trigger. When switching to Binary mode trigger travel is increased although the pull-weight does not. Everything else being equal, a binary trigger should allow you to shoot twice as fast as you normally would. Actually, the BFSIII allowed me to shoot more than twice as fast as I normally would be able to. 

On my first trip to the range to test out the BFSIII I put 150 rounds of mixed ammo through my Z-5K, which is topped with a Trijicon MRO. The trigger pack ran like a champ. However, to do serious testing of this trigger pack would require a serious amount of ammo, and the folks at TulAmmo (otherwise known as The Tula Cartridge Works) helped out by sending me a full case of their 115-grain steel-cased FMJ ammo.

Franklin Armory BFSIII HK-C1
The Franklin Armory binary trigger system all installed in the Zenith Z-5K lower, ready for the pistol to be reassembled. Note the selector lever in the full upright position, which is where to position it when removing/installing the selector.

At the Big 3 East Media event I took my Zenith and the case of TulAmmo off to one of the side ranges and started testing. One, then two, then eventually over a dozen people wandered over, curious to see what full-auto gun I was shooting. Most of them couldn’t believe that what they were hearing was a binary trigger.

Four days later I made another trip to the range with a timer and got some hard data. Without trying to go fast, I could fire my Zenith MP5K clone at 550 RPM with a consistent cadence with the BFSIII in binary mode. Trying to go fast I could increase that to 650 RPM (emptying a 30-round magazine in 2.8 seconds), although my rate of fire would go up and down. A fellow gunwriter with a faster trigger finger could run my gun well in excess of 700 RPM.

Franklin Armory BFSIII HK-C1
The Franklin Armory selector lever in Safe position.

The Zenith MP5K clone doesn’t really recoil backward into your shoulder. Instead, it seriously jumps around in your hands, and a high rate of fire doesn’t make the muzzle climb like a rifle, it just makes it jump faster. (FYI the CZ Scorpion EVO has a very similar behavior). Doing “mag dumps” I observed the red dot of the MRO moving in a rough oval pattern.

Franklin Armory BFSIII HK-C1
The Franklin Armory selector lever in conventional Semi-Automatic position.

The first testing I did involved what I’ll call “two-shot bursts”, pulling and releasing the trigger as fast as possible. All of this testing was done offhand, the way you’re most likely to shoot this gun. The Zenith likes to bounce, and it seems like the second shot of that binary trigger pull heads downrange right when the pistol is in full recoil. At ten yards, the second shot would hit 4-6 inches above the first, usually a little to the left for me, although it wasn’t exactly repeatable because, as I said, the Zenith has a bit of bounce. Still, that second shot is heading out of the barrel roughly .08-second after the first, and I can’t shoot any kind of traditional handgun that fast at all, much less that accurately.

Franklin Armory BFSIII HK-C1
The Franklin Armory selector lever in Binary position. This will fire on both the pull AND the release of the trigger.

The “two-shot burst” seems to be the most practical use for binary triggers in 5.56x45mm AR-15s, as ARs shoot relatively flat and keep those two shots close together. However, extensive testing showed me that because of the Zenith’s bouncy recoil impulse my groups really didn’t expand much whether I was doing two-shot bursts or full mag dumps.

In semi-auto my splits (time between shots) with the BFSIII were .20-.25-second depending on how tired my finger was. In binary mode I could keep my splits consistently under .10 when trying to go fast. Shooting offhand at ten yards my two-shot “bursts” would hit 4-6 inches apart, at fifteen yards 5-8 inches, and at 25 yards ten inches or more. When doing full mag dumps at ten yards at/near 600 RPM I could keep all my rounds in a roughly 7x9 inch oval, and at 25 yards I could usually keep all my rounds on a USPSA/IDPA silhouette.

Franklin Armory BFSIII HK-C1
A full 30-round magazine dump offhand at ten yards at a measured 625 RPM. The group measures 7x9 inches. The Zenith bounces…but is quite consistent.

Whether doing two-shot “bursts” or full mag dumps I consider 25 yards the maximum effective range shooting this offhand in binary mode, but that’s me with this pistol, using this ammo. Using a full-size MP5 clone and/or different (softer recoiling) ammo and I’m sure I could get my groups smaller. But, that’s still putting rounds downrange at slow subgun speeds and being able to hit a man-sized target repeatedly at indoor distances, exactly what the MP5 was designed to do. If you can get into a steady prone or otherwise locked-down shooting position (again with this gun and this ammo) you should be able to keep all your rounds on a silhouette out to 40-plus yards.

Franklin Armory BFSIII HK-C1
Tarr testing the “two round burst” option of the binary trigger at 20 yards, aiming at the center of the silhouette. The Zenith Z-5K jumps around, but the second shot was up and usually a bit to the left.


At the Big 3 East media event we put 850 rounds of the steel case TulAmmo through my pistol in less than an hour, stopping occasionally to let it cool. Franklin Armory provided a small tube of blue Loctite to go along with the binary trigger. I did not use it on the small screw holding the selector lever in place before the Big 3 East massive ammo dump as I had already taken the trigger pack in and out of my pistol several times for photos for this article. After putting 500 rounds or so through the Zenith in half an hour I found the small screw holding the left side selector lever in place was working itself loose. Of course I did not have a small Allen wrench with me. I don't consider this a criticism, they gave me Loctite that I didn't use, I just want you to be aware that the screws if not properly locked down can come loose. Apart from the selector lever screw issue the binary trigger ran perfectly.

Prior to my next range session I liberally coated that screw with the provided Loctite and gave it a day to set up. After another 200 rounds the screw still had not started to loosen. Personally, I think the left-side selector lever should be machined all in one piece with the crosspiece for the ease and convenience of right-handed shooters.

I have seen in several places people saying that it is not possible to outrun the bolt and get hammer follow with Franklin Armory’s unit for the HK. This is apparently due to the cycling time of the gun, not because this unit has something which acts like the second disconnector on the FosTech AR-15 unit. During testing for this article over 1200 rounds fired, neither any of my helpers nor I were able to induce a BFSIII malfunction. The Zenith with the BFSIII installed ran and ran and ran.

Franklin Armory BFSIII HK-C1
1,200 rounds were fired during testing with the vast majority of that being steel case Tula 115-grain FMJ. Reliability was excellent.

Real World Utility?

To answer my own questions: 1. The BFSIII works as advertised, 2. Depending on how fast you can move your finger you should expect between 500-700 RPM, and 3. I think it has serious real-world utility.

The MP5 was designed as a select-fire bullet hose/lead faucet, meant to put lots of rounds fast into bad guys at close range. Unconstitutional laws prevent regular citizens from enjoying their Second Amendment rights to weapons of war. That's the purpose of the 2nd Amendment folks, look it up, the entire Bill of Rights is to keep our own government in line. Franklin Armory’s binary trigger system for the HK rectifies that somewhat, allowing you to legally shoot at slow full-auto-like speeds.

Let’s look at the utility of a brace-equipped MP5 pistol, especially with a binary trigger. In semi-auto it is much MUCH easier to get hits at distance (50-100 yards) than with a standard (Glock/S&W) handgun. With a side-folding or collapsible brace and a 20-round magazine inserted for compactness, you can fit it into surprisingly small bags and backpacks. This allows you to have some serious defensive firepower at your fingertips no matter your social situation. Because it is legally a pistol, you can keep it loaded in your vehicle and can cross state lines without having to fill out any additional federal paperwork, unlike with an SBR.

Pistol bullets are poor fight stoppers…but a hail of them at 600 RPM will work wonders. Plus, the psychological effect on the bad guys of what sounds like a full-auto weapon can’t be discounted. 9mm Parabellum subguns have been popular choices with militaries and law enforcement agencies for decades for very good reasons, and an MP5 with a brace and a binary trigger is a rose by any other name.

As for whether it’s worth the money, I wish the trigger pack was less expensive, but I am a firm supporter of capitalism even when it costs me money, and it’s a hell of a lot cheaper than an actual full-auto MP5. The simple fact of the matter is that Franklin Armory, when it comes to the HK and CZ platforms, is the only binary trigger game in town. Personally I like it so much I’m trying to figure out what I need to sell to afford a full-size MP5 pistol and another BFSIII.


Franklin Armory



SB Tactical


MP5 20-round magazines

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