Back in pre-pandemic times, Mark Serbu of Serbu Firearms brought to SHOT Show 2020 an interesting prototype of a semi-automatic rifle chambered in 5.56×45 mm, which he christened the Diabolus. Now, after about 17 months, we’re given the opportunity to get an update on the development of this firearm, thanks to Mark Serbu’s own YouTube channel.

Serbu Firearms @ TFB

• Mark Serbu Shoots RN-50 Rifle Chambered in .50 Spotter-Tracer
• Mark Serbu Shows How to Make a DIY Rifled Barrel
• POTD: Mark Serbu’s Barrel Test Gun
• [SHOT 2020] Serbu Firearms DIABOLUS Rifle

Serbu Firearms DIABOLUS Rifle – Development Update – Early testing in the desert by SHOT 2020. Great patch! (Credit: Mark Serbu, via YouTube)

Serbu Firearms DIABOLUS Rifle – Development Update

When we first saw the Diabolus (do yourself a favor and check Hrachya‘s excellent article, listed above), it was an early prototype, not yet optimized to actually cycle. The main components shown in the new video were mostly already there, and it was a functional firearm, but just as a manual repeater.

Now we are shown the first shots with semi-automatic functionality and we are offered a quick overview of the elements that allow it. While Mark Serbu openly states that the firearm is still a prototype, as noticeable in a few details, and some tweaks are still needed, the operation looks solid.

Serbu Firearms DIABOLUS Rifle – Development Update – Cycling! (Credit: Mark Serbu, via YouTube)

The operating system of the rifle is a clever mix between direct gas impingement and short stroke piston: an AR-15 style gas block taps gas from the barrel and feeds it through a gas tube to a cylinder closer to the receiver, in this chamber, the expanding gases push a piston. This piston is connected to a rod that transfers the rearward impulse to the bolt carrier group.

Currently, the piston has no return spring, but this does not seem to affect the functionality as, while the bolt closes, the BCG pushes the rod, and therefore the piston, back to its resting position.

Serbu Firearms DIABOLUS Rifle – Development Update – A custom gas block. (Credit: Mark Serbu, via YouTube)

Serbu Firearms DIABOLUS Rifle – Development Update – Gas block disassembling for cleaning… or fondling. (Credit: Mark Serbu, via YouTube)

Serbu Firearms DIABOLUS Rifle – Development Update – Detail of the 2 piece piston. (Credit: Mark Serbu, via YouTube)

Serbu Firearms DIABOLUS Rifle – Development Update – Who wouldn’t want an office that doubles as a shooting range? (Credit: Mark Serbu, via YouTube)

Here below you can find the full video. It’s just a few minutes long and definitely worth a watch, thanks to Mark Serbu’s straightforward style, both informative and entertaining.

What do you, TFB readers, think about this rifle and its peculiar operation? Would you like to see a commercial version of it?