Sig Sauer MCX: Auto-adjusting Gas Regulator or Manually Adjustable?

   10.20.15

Sig Sauer MCX: Auto-adjusting Gas Regulator or Manually Adjustable?

I recently turned a friend of mine on to the SIG MCX as a home defense gun. This gun has been getting rave reviews, and it’s intended to be a successor to the famous MP5 as a quiet, close quarter battle weapon. I think a suppressed MCX with frangible ammo, an Aimpoint red dot, and a Surefire light would be a pretty amazing home defense gun. In fact, it’s hard to think of something better suited for that role.

At any rate, I sent him a link to a Cabela’s page where they’re selling the MCX, but it turns out that the Cabela’s version of the gun has an auto-adjusting gas regulator, whereas the standard civilian version has a manually adjustable gas regulator.

He expressed a preference for the manually adjustable one, but I tried to steer him away and back toward the Cabela’s version. Here’s an edited version of my argument.

To Regulate or Not to Regulate?

There are a lot of opinions about adding adjustable gas blocks to ARs. Our own Major Pandemic is a huge proponent of this mod, but my own opinion is that shooters, especially new ones, should be leery of this.

For readers who aren’t aware of how a carbine’s gas system works, I’ll give a bit of background first.

A gun, like the MCX, cycles its action on every shot because the gas from the bullet firing builds up pressure in the barrel, and some of that pressurized gas is bled off through a little hole in the top of the barrel where it pushes back against a piston assembly that cycles the action. So the more pressure you have building up in the barrel from a bullet firing, the more gas will push on the piston rod and the harder the action will cycle.

When you add a suppressor to a gun, this traps more of the gases in the barrel and raises the pressure, causing the action to cycle harder, which means more noise and more wear. Conversely, when you shoot subsonic ammo in a gun, this ammo produces much less gas. So in a combo of suppressor + subsonic ammo, each cancels the other out and the action cycles normally, i.e. less gas but more capture of the gas that there is means same amount of pressure on the piston system.

It’s important to note that for many guns the action won’t even cycle fully if you shoot subsonic ammo without a suppressor attached. And along those same lines, if you shoot supersonic ammo with a suppressor attached the action cycles really violently, which is unpleasant and causes more wear.

So to sum up, with any given gun there are two variables that you can control that govern how hard the action cycles: the ammo (subsonic or supersonic), and the absence/presence of a suppressor. You have to get that combo correct, because if you don’t then the gun won’t cycle and you have a single-shot weapon.

With a manually adjustable gas system, you’ve added a third variable to keep track of. So let’s say you’re out at the range firing supersonic ammo through your MCX with a suppressor attached, and you throttle the gas back to keep the piston from slamming the bolt back too hard. Then later that night, you hear a noise downstairs so you grab your MCX and a magazine full of subsonic and go down to investigate. Did you to remember to throttle the gun’s gas back up, because if you didn’t you’ll be in for a rude surprise if you start firing it.

Adding this third variable that your brain has to keep track of is considered by many to be a Bad Thing, because if you mess up and you’ve got the gas regulator throttled back at the wrong time and you get into a firefight, you’re screwed. This is why despite the popularity of manually adjustable gas regulators on the 3-Gun competition circuit, you rarely or if ever see such devices fielded by actual military types, except maybe on specialized CQB weapons like the MCX. If you’re soldier, the main thing is that the gun goes bang every time you pull the trigger. How violently it’s cycling or how loud it is is secondary because it has to go bang every time, and you want to take operator error out of the equation as much as you can.

Which MCX?

So, I personally love the idea of a self-adjusting gas regulator, but with the caveat that it has to actually work reliably and not accidentally under-gas the gun. If Sig’s self-adjusting regulator actually works, then in my opinion it’s a better gun than one with a manual adjustment because you’ve just reduced by 1/3 the number of scenarios in which you grabbed the wrong magazine with the wrong ammo and the gun is in the wrong configuration and now it’s not cycling.

Of course, I still can’t answer the question of which MCX to buy because I have no idea how good the self-adjusting gas regulator is. Which is why I turn to you guys to enlighten me. Does anyone have experience with it that they can share? Have you heard any rumors on forums? What’s the scoop?

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