[sc:bcvideo id="3479939283001"]

The Sig Sauer MCX was born as rifle caliber replacement for the Heckler & Koch MP5SD submachine gun. Why? Well, because they could. Submachine guns developed prior to the MCX were hamstrung by technology and materials that limited their reliable operation to pistol cartridge pressure levels. But, rifle power performance in a compact form factor is what we've come to expect as desktop computers are replaced by phones the size of a candy bar.

Ditching the handgun cartridge and running more powerful rifle ammunition called for an updated operating system. The MP5's roller lock, delayed blowback system is fine for pistol caliber loads, but these systems struggle to deal with the higher pressures of rifle caliber loads. While the MP5 is arguably one of the most reliable guns on the planet, you can look at the HK G3 for a hint of how the higher chamber pressure of a rifle round affect roller lock, delayed blowback systems. In a blowback system, the brass case is still filled with chamber gasses at the beginning stage of extraction. HK had to use a fluted chamber in the G3 so the brass has room to expand and contract in order to provide reliable extraction. (Even so, torn cases are not unheard of in the G3.)

In a Stoner system, the gasses are vented through the gas port and the chamber pressure is much lower by the time extraction happens. So Sig Sauer decided that the rotating, locking bolt, as we've become used to on the Stoner AR action, is needed to provide reliable extraction and safely contain the chamber pressures of rifle cartridges.

Looking at the MCX, it bears a passing resemblance to an AR; and it does use AR lowers. But, it's obvious that it doesn't have a typical Stoner action receiver extension or buffer system. While Sig Sauer's Kevin Brittingham wouldn't show me what's going on under the hood during an extended range demo, he did explain that the operating system does have a rotating, locking bolt and very short buffer system.

The gun is a pussycat with subsonic ammo. As you can hear and see in our video, the sound of subsonic rounds impacting the berm 30 yards downrange might be louder than the MCX's report. If you wanted to make the gun any quieter, it seems like you'd need to wrap the action in a pillow. Supersonic ammo was louder, but still comfortable to shoot without earpro and the gun cycled through mags like a bill counter counting singles at the Pink Pony on payday.

We demoed the MCX this week at the Sig Sauer Academy with an integral suppressor made completely of titanium that lightens up the front end of the gun, giving it a much more neutral balance than you'd get when mounting a typical stainless steel or nickel alloy and titanium can up front. Kevin Brittingham, the head of Sig Sauer's military division, knows a thing or two about silencers and he says titanium is much more sensitive to heat than heavier metals and will disintegrate if it gets too hot. That's why titanium cans have been used on bolt action rifles where slower firing schedules don't heat up a silencer as quickly. Some new metallurgy and construction techniques developed by Sig's new silencer division were used on the MCX, above, and Brittingham says the new can is durable enough to last 10,000+ rounds in an assault rifle role.

The MCX is nearly finished. There are few more changes coming before it will get into end-user hands. Sig is going to do some more work on the stock, perhaps some mechanical simplification and weight reduction; and the handguard is going to get some attention. I'm told it's going to get the KeyMod treatment --and the first thing I'd recommend installing are a set of rail panels. Shooting the MCX was a blast, but after a few mags without gloves I was surprised by a little burn. I'm not sure if I gripped the hand guard so tightly that the pad of my ring finger touched the suppressor through a vent, or some prolonged firing got one of the handguard's helicoils hot enough to shock me with a little burn. Either way, it wasn't a dramatic, skin-peeling incident. Just a microsecond distraction.

Aside from a few fore and aft tweaks, Brittingham says the heart of the gun is complete. Running a dozen mags through it on a flat range can't tell us the whole story. But, it does show us that Sig has made some impressive gains in sound reduction and made short stroke piston gun that can cycle 110 or a 220 grain, 30 caliber bullets reliably and interchangeably without the need for a complicated switching mechanism or the added length of a standard buffer system.

The fact that the MCX's little brother, the pistol cartridge-firing MPX, is out in the wild also tells us that the basis for the shortened, closed-bolt operating system is mature enough for public consumption.

It's hard to look at the MCX and not wonder what's going on with the AAC Honey Badger. AAC has been pretty quiet about their gun, and we expected to see and hear more about it at this past SHOT show. Instead, we heard rumors from a reliable source that the Honey Badger will get its grand uncaging upon the civilian market this month at the 2014 NRA Annual Meeting.