At SHOT Shows or other exhibitions, booths of companies like Serbu Firearms are some of the most interesting ones to visit. And the reason is, as a rule, such companies constantly experiment with new ideas and if you are lucky enough, you can meet and discuss their inventions with the actual owners who also oftentimes are the chief designers. This year, I had another chance to meet Mark Serbu and learn about his new creation – a 5.56 rifle dubbed Diabolus.

First and foremost, Serbu firearms are not planning to manufacture this rifle. Instead, they are looking to sell the design to another company. Moreover, there are already companies interested in acquiring the manufacturing rights to produce this rifle.

As Mark told me, the Diabolus is his take on the general AR-18/AR-180 design concept. However, it is significantly different from the AR-18. Probably the two solutions that are most similar to the AR-18 are the recoil spring guide rods that the BCG rides on, and the takedown procedure that starts by pushing the protrusion on the buffer plate to disengage the guide rods from the lower receiver and allow the upper receiver to pivot.

One of the differences of Diabolus from the AR-18 is the use of standard AR-15 bolts, firing pins and barrel extensions which should significantly decrease the manufacturing costs. This, however, led to modifying the guide rods to fit the cam pin and as a result, Mark ended up with a better design which is now being patented. Here is how he describes this designing challenge and the solutions he found:

What I did differently is to use standard AR-15 bolt heads, firing pins and barrel extensions, which of course means the cam pin is vertical instead of the 9 o’clock position.  Because of the small amount of clearance for the cam pin to move between the rods I had to put a small notch in the left one.  While I was at it, I lengthened the notch into a flute that runs nearly the whole length of the rod and then repeated it in 120-degree increments.  So now both rods have 3 flutes on them which reduces weight and drag, and makes it so the left and right rod are interchangeable.

The Diabolus rifle is a gas-operated firearm with a tappet piston located just in front of the barrel trunnion. The gas block, however, is in a normal rifle or carbine location and the gasses are delivered to the piston via an AR-15 type gas tube. In other words, this is sort of a hybrid of short-stroke gas piston and direct impingement systems. Such a system allows to easily build different length gas systems by installing the gas block in the proper location and only changing the gas tube length, without replacing or modifying the piston.

The lower receiver of this prototype rifle is machined out of 7075-T6 aluminum and to make the upper, they used 6061-T6 aluminum tubing. As Mark notes, both receivers can be also made of stamped sheet metal. The lower receiver has a separate replaceable rear piece where the guide rods are locked into. This solution allows switching between different stock mounting options (AR-15 buffer tube, Picatinny rail, or no stock for pistols) in a matter of changing that rear block. There is also a rear urethane buffer block for the BCG to impact on.

The handguard attachment system is also pretty interesting and unique. The actual handguard is held inbetween rear and front caps. The rear cap is attached to the barrel trunnion and it is connected to the front cap via bicycle spokes! The spokes run underneath the actual handguard. This handguard attachment system still allows having a free-floated barrel and according to Mark, it is extremely strong. The handguard itself can be made of a variety of materials (polymer, aluminum, carbon fiber, etc) and I suppose it can be very slimline and lightweight since all it is is a large spacer between the two caps. The handguard design is being patented too.

To me, this is a pretty interesting blend of new design solutions and existing readily available parts. What do you guys think about the Diabolus? Should it become available, would you give it a shot? And if yes, what price range do you think is justifiable for new firearms like this.