Greetings all, we helped develop the BRN-180 for Brownells after we developed the retro BRN-10 for them. We were tasked with adhering as closely as possible to the AR-18 and have it be capable of using a standard AR15 lower. Most companies doing current piston uppers successfully have used the AR-18 as a starting point. The BRN-180 is s short stroke gas piston upper, with a trunnion that replaces the standard barrel extension in an AR15. The BRN-180 has a recoil modifier attached the the rear of the upper. Our upper uses a recoil modifier that threads into the buffer tube thread on a standard AR15 lower. This allows the recoil to be absorbed by the buffer tube loop on the AR15 lower (kudos to Eugene Stoner).
The FM15 Gen 2 is a direct impingement upper, with greatly reduced mass (small carrier, no buffer or buffer tube). The reduced mass and design allow for unique recoil impulse that is very flat shooting. We started developing the gun several years ago and spent the last 3 months of production testing mainly shooting suppressed. We found they run well suppressed using anemic steel cased ammo well (imagine that), but had to increase the gas port a little to run anemic ammo unsuppressed.
There is a nested recoil spring that allows us to have an integrated recoil system in the upper, allowing consumers to use a folding adapter. The outer spring is 12 plbs and coiled clockwise. The inner spring is 8 plbs and is coiled counter clockwise. The Bolt carrier is supported by the upper receiver bore, and also the gas key is unique, and there is a recoil rod that goes through a hole in the gas key, allowing the gas key to ride on the rail, and guaranteed there is no carrier tilt.
There is an improved round cam pin, and a pretty interesting gas block design. We drill the gas block port from the top. When we assemble it, we test fit the gas tube in the gas block by installing it upside down. Then we can look through the hole at the top of the gas block and confirm the ports are aligned. Then we remove the gas tube and slide the gas block on the barrel. We look down the gas block and confirm alignment. once we confirm that, we pin the gas tube to the gas block, then install the gas block on the barrel.
We keep a very tight fit between the gas block and barrel, and use (2) 1/8" pins to attach it to the barrel. There is an interlocking feature between the barrel and gas block, to guarantee good alignment. We use a gage pin through one of the 2 holes in the gas block to guarantee alignment before driving in the 1st 1/18" pin.
The barrel has 6 flats cut on it at the chamber end of the barrel. pretty much every company making Ar15's uses a 10-20 ton press to clamp down on a round barrel, and use 125-150plbs of torque to install the barrel extension. We use the 6 flats machined into our barrels, and slide it down into a 1" plate with a mating shape cut into it. Then we slide the barrel into the plate, and torque the barrel extension, this ensures we dont crush the soft barrel material when installing a barrel extension.
The upper can be dissembled with a punch, but I like to use a screw driver (seems easier to me). There is a 20 plb recoil spring ready to jump if you are not careful, so wear eye protection when disassembling them. The upper and handguard are seamless and mimic a monolithic upper. there is an interlocking feature between the upper and handguard, that guantees the barrel nut wont come loose after the handguard is installed. The handguard clamps down on the barrel nut, and interlocks with the upper.
The barrel nut and handguard are kept to a tight fit, and the barrel nut is long to provide good torsion support over the length. It is held in place with (2) torx socket head cap screws. They allow the handguard to be removed many times without stripping the screws, and make torqueing the screws very simple.
Another unique feature is the taper lock barrel nut. The barrel nut and barrel are made in the same Okuma Twin Spindle lathe. There is a taper on the inside of the barrel nut, and the on the outside of the barrel (in front of the chamber). When the barrel nut torques down, the load is distributed over a large surface area, and this helps with keeping the concentricity between the upper and barrel.
Where the BRN-10 has a 2 position adjustment (suppressed and unsurpressed), we found the Gen 2 ran well suppressed so we opted to offer a suppressor recoil spring in order to offer the Gen 2 at a better price point.
The guns run well suppressed, but if people ping us we generally send them a spare suppressor recoil spring. The main benefit of a Supressor recoil spring is it slows down the carrier a little.
There were challenges getting the BRN-180 and gen 2 to market. Any time you launch a new product, the 1st 1000 customers who try it, are critical to making improvements to the gun. We did extensive testing on the 2 platforms, but consumers using varied combinations of ammo, parts, magazines really allow you to bring a more mature product to market.
How important is running steel cased ammo to consumers?
We have found steel cased (and even some Russian brass) is anemic. Some ammo is about 15% less fps using a chronograph, thats a big difference, which is why many companies have guns that are over gassed (so they cycle w anemic ammo). I think its harder to make a gun run steel cased 5.56 than to make one run suppressed. With our 9mm platform, they love steel cased ammo, but a blow back gun is very different than a DI 5.56
Full disclosure, we have shipped around 1000 units between Aim and Brownells and have found 2-3 units that appear to be undergassed. We resolved those customers concerns, but have not been able to get the uppers returned for us to evaluate. We had used an off the shelf gage to check the gas tubes, but it did not identify gas tubes that were a little undersized. FM had been getting gas rings from CSS, and had not been inspecting them. We have already added a step to qualify the diameter of the gas tubes and gas rings and have not see any issues since. We Likely anemic ammo may have also played a part in this, I will follow up on this when I get those units back.
Hope this info is helpful, I will do my best to keep an eye on this post, please be patient as we are jammed up running the shop etc. Best way to reach us is email
[email protected] we offer customer service support 7 days a week too.
