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Posted: 7/28/2025 1:55:45 PM EDT
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After the Rare Breed legal settlement with the ATF and FRT triggers I wanted to realize my dream of having something as close to a loud (un-suppressed) select-fire shorty Hk416 as possible. I decided to go the Brownells BRN-4 route to keep the price down, but after the headache of getting the thing working I think I may have been better off just buying a PD trade in Hk upper and calling it a day. Anyway, here are the steps I took to get my 10.4" rifle to run smoothly un-suppressed with an FRT trigger (Delta Team Tactical 3-position, now sold on Rare Breed's website). There are two issues you'll run into with FRT triggers: gassing (cycling power) and bolt bounce. First, proper gassing. Because the FRT trigger steals a small amount of energy from the bolt during cycling to reset the trigger you'll want err on the side of over-gassed instead of under-gassed. Brownells made their 10.4" BRN-4 uppers with a small gas port (~1.55mm) and a vented gas block. While this may run great suppressed, when un-suppressed it's barely enough to cycle the upper at all with high pressure 5.56mm ammo and a standard strength carbine string and maybe an H-buffer if you're lucky. No luck at all in FRT mode or with the Hk416 enhanced spring + buffer. I tried the Nefarious Arms piston, which helped a little. The NA piston has a slightly longer nose which gives the piston a longer pushing impulse before letting gas escape out the vent on the front of the gas block. This gave me a more reliable bolt lock-back with a standard carbine spring with stiff 5.56mm ammo, but still not enough for FRT mode. After reading more through the HkPro forums I found this thread (link ) which gave advice for gas port size on the BRN-4 for reliable function. I bit the bullet and decided to re-size the gas port to 1.8mm. This meant cracking the flash hider off which I'd timed and locked with rockset before even shooting the upper. Don't be me. Amway, I bought a Nefarious Arms HK 416 Gas Block Assembly Tool and some 1.8mm drill bits. After taking off the flash hider and removing the gas block with the NA tool and a good set of punches, I drilled out the gas port to 1.8mm. This was actually pretty easy--I drilled it by hand with a Milwaukee 2801 cordless drill I picked up from Amazon. I got a cheep aluminum cleaning rod and stuck that down the barrel while drilling the gas port so I'd have a backstop and not ding the opposite side of the bore after drilling through. The existing gas port guides the bit so it was pretty easy to keep the hole straight. There are a couple youtube videos about drilling gas ports in general and they're pretty good guides. Then I re-assembled the upper. The second issue is bolt bounce. As the bolt cycles, it resets the trigger during its backward travel. The FRT trigger has a small trip at the rear of the trigger pack which will keep the trigger from being pulled again until the bolt finishes its forward cycle and is back in battery. The ledge on the bottom of the bolt (see the difference between an auto bolt carrier and a semi-auto carrier) accomplishes this when it hits the trip as the bolt moves into battery. A problem I ran into while trying different buffers and springs was that light buffers would allow bolt-bounce to happen. This is when the bolt fully closes into battery and trips the FRT trigger allowing the hammer to begin to fall, but the bolt bounces back up out of battery. The hammer smacks the bottom edge of the carrier (missing the pin and not firing out of battery, thankfully) and then follows the carrier as the BCG settles home. The result is a dead trigger--you'll have a live round in the chamber, but an un-cocked hammer. The H, H2, and H3 buffers have free-floating tungsten weights that counteract bounce by adding a slight delayed smack to the BCG and keep it in place. The HK buffer has powdered tungsten inside instead of discrete shake-weights like H1,2,3 buffers, which does he same thing (maybe better?). Anyway, if you find yourself using a lighter buffer because of under-gassing, you'll probably have the bolt-bounce issue (the H-buffer and standard carbine was causing bolt-bounce dead trigger with the NA piston before I drilled the gas port). Once I got the gassing corrected I switched to the Hk416 red spring (stronger than normal carbine springs) and the Hk416 buffer (between H2 and H3 weight, with powdered tungsten). That fixed the bolt bounce dead-trigger issue. With my current setup (1.8mm gas port, NA gas piston, Hk416 buffer and red buffer spring) My rifle is running FRT in short (3 - 5 rounds) and long (15+ rounds) bursts reliably. I ran about 80 rounds over 3 magazines and didn't have any bolt-bounce dead triggers or under-gassed FTF or FTE issues during my last testing session. I did have one instance at the end of a long burst where it did not lock the bolt back. I'm getting an ejection pattern between 3 - 4 o'clock. I played back some video in slo-mo and timed a long burst and counted the rounds and came up with about a 700 RPM firing rate. This is lower than the 850 RPM the Hk416 should have. So all taken together (one bold-hold failure, 3-4 o'clock ejection, and RPM) this indicates it's still not gassed optimally, but at this point I'll probably take it as is instead of opening up the port even more. Down the road I'll probably pick up an MR556 A4 if I can find a used one for a good price. To get the A4 working with an FRT you'll have to swap the Hk BCG with a BRN-4 version, which has the auto carrier rear shelf, no firing pin safety, and the correct cutout for the barrel extension tab to work in an MR556 upper. The A4 has an adjustable gas block as well as gas screws with different sized vents you can buy to further tune the gas system. Another option I'm considering is just selling the whole upper and slapping the $$$ for an old Hk416 PD trade in with a non-ported gas block. Those things have a pretty stiff cycling impulse bordering on excessive. That would get the FRT going as well as bring up the cyclic rate. Anyway, I got a clone lower from Gigliotti Rifleworks which I'm still putting together. Once I get it SBR'd I'll get some pictures up. (Currently I'm just using another of my standard AR15 SBR lowers.) |
The law perverted! And the police powers of the state perverted along with it! The law, I say, not only turned from its proper purpose but made to follow an entirely contrary purpose!
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Try it with a suppressor? My first thought is that a cheap contemporary design, polonium, turbo, or one of WPA’s new cans, would solve the gassing issue without needing to drill out the gas port. It’s also surprising to hear it’s still undergassed with HK spec gas port… The 416 has always been stupidly overgassed and oversprung, I didn’t think FRTs robbed the BCG of that much energy. |
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TX, USA
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First, I appreciate the time you took to do this write up. It gives me some ideas as I contemplate building one of these. Thank you I'm just guessing but it seems like a traditional suppressor (to increase gas pressure) would have been cheaper than all the things you did to make it work. AND you would then have a can to use on other rifles. A cheap can like a Polonium K would probably work fine. I believe they are legal in Utah. |
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OR, USA
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Originally Posted By LawGotz: Any word on if Brownells is going to do more BRN-4 uppers? Are they not on their website, or are they just out of stock? I know that one of the local shops near me has two of them in stock, they arrived about 2 weeks ago, and an assortment of Geissele Blemula handguards to go with them. I bought both of my uppers from the same shop when Brownell's was out of stock on their website last year. They are solid uppers for the money, they run very well. |
Joined:
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OR, USA
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Stock is super low- all stripped uppers out of stock. A few no handguard 14.5 and 16 length uppers available in limited barrel weights. I am hoping they do another run of 10.4 length uppers. But maybe they are not doing anymore? I asked a Brownells customer service rep and they said they had no eta of more BRN-4 stuff. |
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