SIG Air Rifle Secrets – Part Two, The Firing System

In the second part of Hard Air Magazine’s SIG air rifle secrets series, we look at the firing system inside these guns.

MPX And MCX Firing System

First let’s look at the MCX firing system. That for the MPX is identical.

SIG Air Rifle Secrets

Yes, with the right sideplate off, the action looks to be overwhelmingly complex. That’s one of our SIG air rifle secrets!

Remember that the 88/90 Gram CO2 cartridge screws into the nut at the rear of the action (the left of this photograph). The CO2 then passes through three right-angle bends and a brass tube to make its way to the valve.

In the photo above, we see how the external hammer surrounds the valve. The hammer spring is enclosed in a plastic “cassette” behind it. Their location means that the CO2 has to be taken to the valve by the CO2 tube above those parts.

Now let’s strip out the firing system. This makes it easier to see how these link together.

SIG Air Rifle Secrets

Note that all SIG long airguns are notable for an almost complete absence of O rings! These staples of the CO2- and PCP-airgun world are nearly absent from all these models. However split rings are used in a one or two places. That’s something I’ve not seen before!

Now for an exploded closeup of the valve itself. The CO2 enters the valve body from the top, about half way along its length.

SIG Air Rifle Secrets

Let’s take another look at that CO2 tube. It’s very thin – just 3mm outside diameter.

Of course, this was originally designed to be a CO2-powered airgun. So the pressure of the gas from the 88/90 Gram cartridge would flow through that tube and keep the pressure high at the valve.

The CO2 cartridge acts as a large reservoir at (relatively) constant pressure. The diameter of the tube is therefore not a serious issue, particularly as the action fires only once per pull of the trigger and the gas draw is thus constrained naturally.

SIG Air Rifle Secrets

Canebrake And Virtus Firing System

CO2-powered airguns operate at a maximum pressure of about 1,100 PSI. That’s the maximum pressure that CO2 will achieve under reasonably normal temperature conditions.

This has encouraged multiple airgun designers to design regulated PCP airguns based on CO2 prototypes using 1,100 PSI as the regulated High Pressure Air (HPA) pressure.

(I know, I’ve designed PCPs on that basis myself based on the QB79 CO2-powered air rifle).

So it was logical for SIG to consider a regulated PCP version of their CO2-powered long guns. In this case, up to 3,000 PSI of High Pressure Air is contained in the HPA bottle. This is then regulated down to 1,100 PSI before it enters the gun.

This is a close-up of the Virtus’ regulator.

SIG Air Rifle Secrets

But, there’s a BIG BUT! That’s another of our SIG Air Rifle secrets…

As HPA is “thinner” than CO2, it requires a space – called a plenum – in which sufficient regulated air can be stored before a shot takes place. If the amount of HPA is insufficient to fill the valve and propel the pellet out of the barrel, that exciting new PCP will not work correctly.

The MPX/MCX CO2 tube does not have sufficient volume to act as a suitable plenum. Also there’s nowhere else in the gun to make sufficient space.

So, SIG had to design a completely new gun – while looking very similar to the MPX/MCX – to incorporate the required plenum. You can see it in the photo below. Now the gas tube has increased to 10mm O/D!

SIG Air Rifle Secrets

Zooming-out, we can see that the remainder of the action is similar to that of the MPX/MCX. Yes, there are some changes in the valve porting. The hammer spring is stronger, too.

SIG Air Rifle Secrets

More obviously, the hammer of the Canebrake/Virtus is much heavier than that for the MPX/MCX. That makes sense given the primarily PCP application. In addition – as you can see below – the designers also added an adjustable striker to the hammer. This enables the gun to be tuned during assembly.

SIG Air Rifle Secrets

As many of you will know, Pyramyd Air makes a “PCP upgrade kit” for the SIG CO2-powered long air rifles. This comprises a regulated HPA tank, buttplate and adapter to allow it to fit the threads used by the CO2 guns for the 88/90 Gram cartridges.

It should come as no surprise to conclude that this HPA upgrade kit will work very well with the Canebrake. With the MPX and MCX, hummmmm…

So, if you plan on starting-out with a SIG CO2-powered long gun and plan (or hope) to upgrade to HPA power in future, HAM’s SIG air rifle secrets tell you that the Canebrake will be your best choice.

If you plan on staying with CO2 propulsion, the MPX and MCX will be just fine!

Check out the other parts of this series from these links: Part One. Part Three. Part Four.

SIG Sauer MPX Pellet Rifle Gen 2, Black 0.177
SIG Sauer MCX Pellet Rifle Gen 2, Black 0.177
SIG Sauer MCX Rattler Canebrake CO2 Pellet Rifle 0.177
Sig Sauer ASP MCX Virtus PCP Air Rifle 0.22