Today we’re taking a closer look at one of ARES’ newest additions to their M4 line-up: the Octarms x Amoeba Pro M4 KM9. The KM9 is a mixture of their existing M4 and the Amoeba Pro series featuring a new Octarms free-float rail and, of course, their EFCS Electronic Firing Control system.


The KM9 comes packed with quite a few features, to name a few:

  • Amoeba Pro stock and pistol grip
  • Octarms 9″ free float KeyMod rail
  • New 14mm CW flashhider
  • New flip-up sights
  • Sling plate with QD sling mount
  • New flat trigger
  • Fake bolt with functioning boltcatch
  • EFCS Electronic Firing Control System
  • Quick Spring Change system

As always, let’s take a closer look at all the externals first.

The KM9 features Ares’ new Amoeba Pro stock and pistol grip which remind me a lot of the Magpul ones. Each Octarms x Amoeba Pro M4 model is available in tan or black color versions. The stock features a rubber butt plate on the rear end and a simple sling mount on the lower portion. The stock can be adjusted via the push button inside the cutout section or via the pull button, that is facing forward on the angled section. The pull button is also used to take the stock off the tube.




The stock looks pretty good actually and includes the common modern design features such as a rubber stock plate, slim profile, a sling loop. Another nice features would have been a QD socket for attaching slings.The stock has no battery compartment due to its slim profile, the battery is stored inside the stock tube and that also means you have to take off the stock to insert or remove the battery. I used a RWA 1200mAh 3s LiPo battery and a Turnigy nano-tech 1300mAh 3s LiPo that fit perfectly into the stock tube.

One issue the stock has is its wobble. As Ares is selling those stocks as a accessory, they designed the stock to fit on as many third-party stock tubes as possible. They made the inner diameter of the stock a little wider to ensure compatibility, even though their stock tube is slimmer.

Check out this video below how much the stock wobbles. You can easily fix the wobble by adding some strips of electrical tape to the stock tube. If you’re adding these strips on the rear end section of the tube they won’t be visible.

Ares also added a QD sling mount plate to the receiver/stock tube. The stock tube can easily be taken off by unscrewing the ring and lifting up the stock plate so that the tube can be unscrewed. It is necessary to remove the stock tube in order to use the Quick Spring Change system. Being able to access the QSC system by simply removing the stock tube is great and a lot of manufacturers are already copying the system Ares is using for almost 8 years (they’ve actually been the first manufacturer to utilize this system).


The pistol grip is quite comfortable in terms of ergonomics. It features two anti-slip textures on the front and rear. The grip feels pretty slim for an AR grip, not as slim as a Systema but way slimmer than your average Airsoft AR grip. The finish of the anti-slip inserts is good although there’s some sort of debris left from the casting.


The motor plate features a skeleton design for better airflow and cooling of the motor.


The Grip and the stock feature various marking, including part numbers and the Amoeba logo and name.

The lower receiver seems to be the standard AR receiver Ares is using on their M4 models. The upper receiver looks a little bit different and features a octagonal front rail mounting platform to fit the shape of the rail. The receiver is cast and not machined. The black color is painted and has some flaws here and there. I’m a little bit disappointed by the quality of the finish as I’ve seen Ares doing better on the models I got my hands on in the past.

The polymer trigger guard features small textured sections where your fingers can rest.


The flash hider is mounted on a 14mm CW thread and looks cool. It reminds me of the Surefire 556 SOCOM series. You can easily remove the flash hider and add whatever model you want or you can add various suppressors that feature a 14mm CW thread. The clockwise thread is quite uncommon these days and it can be hard to find matching third-party accessories.


The flip-up sights are made out of polymer and have a very low profile. They can be flipped up and down without having to push/press any buttons. The sights can also be adjusted. The rear sight has only one circular sight hole, and that is quite small – like the smaller one of the typical AR iron sights. But to be honest, no one’s really using the iron sights anyways.

The “Octarms” KeyMod rail is CNC-machined out of 6063 aluminum alloy and very lightweight. KeyMod sections are available at the 3, 6 and 9 o’clock position. The build quality is very good and it seems to be quite durable.


The rail is mounted to the upper receiver with 6 hex screws and not to a barrel nut as usual. Ares also sells the rail as an aftermarket version that includes a matching barrel nut.


Using the upper receiver to mount the rail isn’t the best solution, at least not how Areas realized it. The barrel nut is screwed to the inside of the upper receiver and you can’t really tighten the nut all the way down because of the thread ends at some point. The nut stops when the thread ends and the outer barrel still has a tiny bit of play you can’t really get rid of. A solution would be to use some washers to get it tightened down securely.

The outer barrel features a fake gas block and gas tube.


On the right side of the receiver sits a small plastic part that is used for the functioning bolt catch. It’s not screwed into the receiver but rather just sits inside a small cutout section. Be sure not to loose this part when disassembling the gun – I almost did.




The gearbox features a simple quick spring change system. The spring guide can be removed by using a hex key and turning it 90°.

The hop-up used in the Ares Octarms KM9 is basically a standard V2 hop-up unit that uses a big rotating dial instead of multiple gears. This kind of hop-up is becoming the standard for V2 hop-ups more and more. It’s air-tight, well built and uses a standard-type bucking.

The motor is described as a “high torque” one but the magnets are quite weak for a “high torque” type. It has no markings on it and no decals. A problem I encountered when shooting the KM9 for the first time: I plugged in my battery and did some blank test shots, filled up the magazine and turned on the chrono. After inserting the magazine and pressing the trigger, the KM9 was dead. I had no idea what the problem was as it was working well two minutes ago. I heard a strange noise inside the gun, a noise that sounded like there was an electrical short or something happening. I took the gun apart and saw, the the plug of the motor cable was so loose, that the vibration of the test shots made it come off the motor. I took a plier and tightened the connector a little bit so that it would sit nice and firm on the connector pin.




Ok, now it’s time to take a look at the most important feature of the gun: the Ares EFCS (Electronic Firing Control System) and, of course, the gearbox and its internal parts.


The EFCS is quite a one-of-a-kind system and is very similar to the one Systema is using in their PTWs. The EFCS features an active breaking mosfet and a two magnetic sensors that control the piston cylce (cycle completion) and the firing mode. The EFCS unit is also programmable with an external unit. You can program various shooting modes and the battery voltage / lipo cells.

The EFCS version built into the KM9 is the so called “EFCS Advance Circuit” – a newer variant that can withstand higher voltages (3s LiPo as a standard) and currents.

We’ll get into all the details step-by-step.

The first thing you’ll notice is the small sensor that is placed in an 90° angle to the circuit board. This is a magnetic sensor. The sector gear features a small magnet that rotates past the magnetic sensor and so the circuit board knows where the sector gear is positioned. The EFCS features cycle completion, which means that every time you pull the trigger, the sector gear, thanks to the active-breaking mosfet, stops at the very same position, in semi as well as full-auto mode. So basically that’s what the magnetic sensor is there for.

Due to the small magnet inside the sector gear, it’s quite hard to replace the gear set with an aftermarket one. You’d need to insert the magnet into the new gear set, which is quite a pain as there is no standard for the size of the holes. I saw one video on Youtube where a guy used a G&P sector gear that he was able to insert the magnet into.

A better solution would be to insert the magnet into the delayer clip as you could use the delayer clip on other sector gears too. Just for the sake of upgrades.


The firing mode is also controlled via a magnetic sensor. The selector plate features a small magnet and the circuit board features another magnetic sensor on the back (in the picture below, right next to the small magnet inside the selector plate). This means that there’s no need for a cut-off lever.


Ares says, that the travel of the trigger can be adjusted but to be honest, I’m not sure how to do so. There’s nothing written in the manual about that. The trigger mechanism uses a micro switch that is soldered to the circuit board. It’s quite possible that the push button of the micro switch can be adjusted / moved up and down so that the trigger needs to travel less but I’ll need to take the gearbox apart and check for this again.

As you can see on the photo below, the rubber seal is a bit misaligned and the circuit board should be dissembled and repositioned to ensure correct weather sealing.


The cycle completion and the magnetic selector plate aren’t the only features of the EFCS system. The circuit board can also be programmed with an external programmer unit (available separately) for various shooting modes:

  • Safe – Semi – Auto
  • Safe – Semi – 3rds Burst
  • Safe – 3rds Burst – Auto
  • Safe – Semi – Semi

Another feature of the EFCS is the auto power-off feature in case any internal parts break / fail.


As you’ve already guessed, the gearbox shell is proprietary and can’t be replaced with an aftermarket one and you also can’t place the EFCS circuit board into any other V2 shells. At least not without heavily modifying the shell. However, the EFCS gearbox is fully compatible with any AR-style guns that use a standard V2 shell and a standard AR receiver.




Now that we’ve taken a closer look at the EFCS system it’s time to check out the internal parts of the gearbox.

One thing you don’t see that often in a V2 gearbox is a motor mounting plate. This feature probably adds some stability and is a nice feature nevertheless.

I can’t say much about the gear set as it looks rather standard. Ares advertises the gear set as a “high strength” one.

The shell features 8mm ball bearings for the bevel gear and 8mm bushings for the spur and sector gear.


The spring guide is made out of steel and doesn’t feature any bearings. The spring is quite powerful and the chrono readings were between 437 and 433 feet per second straight out of the box. For some fields this gun might be a bit too strong and would require a different spring. Thanks to the quick spring change system, changing the spring takes only a couple of minutes.


The cylinder set is well made and the tolerances are very low. the compression is good. The only negative thing is that the stainless steel cylinder is a full type and therefore the barrel/cylinder volume ratio is a bit over the top, somewhere around 3.6:1 (1.5:1 to 2.5:1  would be a good ratio). Considering the barrel length, the cylinder should be a matching ported one (probably a 3/4 one).


The polymer nozzle features an o-ring.


Standard polymer cylinder head with a flat rubber pad.


The polycarbonate piston features full metal teeth with the penultimate tooth being shaved off to ensure that the sector gear doesn’t interfere in case the AOE is off.


Polycarbonate piston head with vent holes.




The KM9 is a serious contender for the Amoeba series. The gun is very well built and quite solid, except for the little stock wobbling issue. Although it can easily be fixed, it’s a little downer in my opinion. The finish of the receiver and various other metal parts has some flaws here and there and there’s some room left for improvements. Maybe I’m spoiled by the quality of their polymer frames that most Amoeba guns feature.

The KeyMod rail adds a modern look to the KM9 and built quality is excellent. Ares recently released a whole lot of KeyMod accessories like RIS panels, hand stops, grips etc. that can be added to the rail.

The internal parts are very well made, the cylinder kit has almost no tolerances and FPS fluctuation is quite low. A little downer is the type-0 cylinder. With the short inner barrel (~260mm) the volume ratio is around 3.6:1 which visibly effects the accuracy of the KM9. Although I was able to hit targets at a distance of 75m (which is the longest distance I can test at the moment and quite impressive for such a short barrel), the groupings were off by several meters and I wasn’t able to repeatedly hit a man-sized target. The hop-up works great and I only had to turn it up a little bit to achieve my desired trajectory.

The EFCS system is working very, very well and I really love the cycle completion and the electronic trigger. Again, the only little downer here is the incompatibility with third-party gear sets. Yes, Ares also offers different gear sets for the EFCS gearbox but each player has his own preference when it comes to gear ratios and brands.

Available for a retail price of around 279USD, the ARES Octarms x Amoeba Pro M4-KM9 is actually a bang for the buck with room for small improvements.


  • EFCS system
  • Quick spring change system is easily accessible
  • Very sturdy without any wobbling (except for the stock)
  • Well built Octarms KeyMod Handguard
  • Low profile, slim flip-up sights
  • Cool-looking flashhider
  • Quality internals
  • Slim pistol grip
  • Powerful


  • Wobbling stock
  • Finish of receiver and various metal parts
  • Clockwise barrel thread
  • Type-0 cylinder
  • Changing the gear set can be a challenge
  • Manual missing information for unexperienced users

areslogo1I’d like to thank ARES for sending the test model and giving us the opportunity to review of their products.