by Tom Gaylord
Writing as B.B. Pelletier

Sig Sauer MPX sub-machinegun

Sig Sauer MPX.

Part 1

This report covers:

  • How to load the MPX
  • Orient the magazine by the magazine door
  • Removing the belt
  • Load the belt
  • Feeding the belt into the magazine
  • Sig Sauer Match Ballistic Alloy pellets
  • Crosman SSP pellets
  • Air Arms Falcon domes
  • Does firing fast lower the velocity?
  • Shot count
  • Trigger
  • Evaluation so far

Today we will look at the velocity of the MPX from Sig Sauer. I called it a sub-machinegun in Part 1 and a reader pointed out that subguns are full-auto. This one isn’t, so I erred in using that term. Sig doesn’t use it anywhere. They call it a semi-automatic air rifle, which is correct. I will use that term from this point on.

How to load the MPX

Today is the day we look at velocity, but before we do that, I told you I was going to rewrite the magazine loading instructions, to make them clearer. The manual doesn’t distinguish what is right and left in the instructions, yet they refer to the left and always feeding the magazine from a clockwise direction. Unless the owner knows how to orient the magazine, left, right and clockwise have no meaning.

Orient the magazine by the magazine door

There is a door on one side of the magazine. If you open this door and hold the magazine in your left hand so the open door points to your left, the magazine will be correctly oriented for everything I am about to say.

02-03-16-01-Sig-Sauer-MPX magazine orientation
If you hold the MPX magazine in your left hand like this, everything that follows will make sense.

The belt is now accessible through the open door on the left side of the magazine. If you push up on the belt, it will rotate freely in a clockwise direction from your point of view. This is the only safe direction to move the belt. The manual warns that belt movement in the other direction can damage the magazine. As the gun fires and feeds, this is the direction the belt moves.

Removing the belt

The belt ends with two black pellet chambers on either end. Move the belt around in the magazine clockwise until these 4 black chambers are seen in the open door. Pick the bottom 2 black chambers out of the magazine and pull the entire belt out. It should move easily and freely. Now you can load the belt.

Sig Sauer MPX remove belt
Pull the belt out of the magazine from the bottom, like this. This keeps the belt moving in a clockwise direction that won’t damage the magazine.

Load the belt

Place the belt on a table with the metal ends of the chambers on the bottom. Insert a pellet, nose-first into the top of each chamber and press it flush with your thumb or finger. Once the belt is loaded (all chambers do not have to be loaded for the gun to function) use the pellet seating tool provided with the gun to push each pellet as deep into the chamber as it will go. The tool will stop at the correct depth. When all pellets are seated the belt can be fed into the magazine.

Sig Sauer MPX load belt
Insert a pellet into each chamber of the belt and press flush with a finger.

Sig Sauer MPX seat pellets
Used the seating tool to press each pellet as deep into the chambers as they will go.

Feeding the belt into the magazine

The belt can only be inserted one way. Hold the end of the belt in your left hand with the metal portion of the chambers facing you and the tabs of each chamber pointing to the left. This is the way to insert the belt into the magazine.

Sig Sauer MPX hold belt
Hold the belt in your left hand with the chamber tabs pointing to the left and the metal portion of each chamber facing you, like this.

Now pick up and hold the magazine in the same orientation as before — with the door open to the left. Insert the belt and push it upwards, into the magazine. It will go in easily once the tabs align with the clearance track in the magazine. Keep pushing the belt up and watch it go over the top of the mag in a clockwise direction as it is fed in. When the entire belt is in the magazine you can always rotate it in this same clockwise direction at any time.

Sig Sauer MPX load belt
Insert the belt into the magazine with the tabs to the left like this. Push the belt in and up, so it goes over the top of the magazine in a clockwise direction. It should feed easliy once the tabs are aligned with the magazine raceway.

That is how to load the belt and also how to both remove it and insert it back into the magazine. Once you get the hang of it, you won’t have to follow these steps exactly as they have been described. This is just to get you up and running at the start. Now we’ll look at the velocity. For the first part of the test I waited a minimum of 10 second between each shot to let the gun warm back up. The room was 70 degrees F when these tests were conducted.

Sig Sauer Match Ballistic Alloy pellets

I started the test with Sig Sauer Match Ballistic Alloy pellets that weigh 5.25 grains. They fit the chambwers of the belt very tight. They averaged 532 f.p.s., with a spread than went from 525 f.p.s. up to 540 f.p.s. That’s pretty tight for a CO2 action airgun, but the velocity is lower than the advertised 600 f.p.s.

Crosman SSP pellets

To give the MPX every opportunity to achieve its advertised velocity I next loaded 10 Crosman SSP pellets. These lead-free pellets weigh 4.0 grains and seated in the chambers much easier than the Sig pellets. They averaged 598 f.p.s. with a spread from 581 to 608 f.p.s. 4 shots were above 600 f.p.s., so the MPX certainly does meet the advertised velocity of 600 f.p.s. It only does so with lightweight pellets, but you have to understand that advertised velocity is for product liability purposes. It represents the top velocity the gun can achieve. You may not think that way, but a company selling airguns has to, because they might have to defend their product in court someday (i.e. how dangerous it is, should it be used by adults only, etc?).

Air Arms Falcon domes

To get a realistic idea of the velocity using real-world pellets that an owner might select, I tried the Air Arms Falcon domed pellet. This 7.33-grain dome is light weight and made from pure lead. It’s usually very accurate in most lower-powered airguns.

In the MPX Falcons averaged 471 f.p.s. with a spread from 461 to 489 f.p.s. This is the sort of velocity that can be expected from the airgun on an everyday basis.

Does firing fast lower the velocity?

Next I wanted to see how shooting very rapidly affects the velocity. You know that’s what owners are going to do. So I loaded 4 Crosman SSP pellets. I fired the first one to see if it was still in the same range and it registered 598 f.p.s. Then I removed the magazine and dry-fired the gun 30 times to chill it. When that was finished the gun felt colder to the touch. Then I inserted the magazine and shot the next 3 SSP pellets with a 3-second pause between shots. The velocities were:


What this shows is the gun does slow down when fired rapidly, but it also recovers quickly. From what I see there shouldn’t be a problem firing as fast as you want — especially on warm days.

Shot count

I know everyone wanted to know how many shots to expect from one of those 90-gram CO2 cartridges. I was using a cylinder provided by Sig Sauer, but Pyramyd Air doesn’t carry them, so I linked to a cartridge they do carry. The results should be pretty similar, as these cylinders are all filled to the same standards, though each one will vary from all others by slight amounts.

To test the shot count I loaded a belt with SSP pellets, then shot the first one for a test of the remaining power. The velocity was 583 f.p.s., which is about where it should be.

Then I removed the magazine and fired 50 blank shots as fast as I could pull the trigger. The gun was very cold after this, so I let it warm up for about 5 minutes. Then I inserted the magazine and fired the 118th shot on this CO2 cylinder. It went out the muzzle at 554 f.p.s. I can’t be sure the gun was fully warmed up after the 50 shots, but this does simulate shooting the gun a lot at one time.

Then I removed the magazine and fired another 50 blank shots. After another 5-minute rest to warm the gun I inserted the magazine and fired shot number 169 It went out at 500 f.p.s. I also noticed there was no fog of CO2 issuing from the empty magazine port about halfway through the 50 blank shots this time. That fact plus the lower velocity tells me the liquid CO2 is used up and the gun is now running on residual gas pressure.

I removed the magazine and fired 25 more blank shots. Then I let the gun rest another 5 minutes. Then I inserted the magazine and fired shot number 195. It left the muzzle at 448 f.p.s. and that was where I decided to stop the test. Could I have continued? Certainly. But at some point I risked getting a pellet stuck in the barrel.

My test was artificial. It only approximates how the gun will perform in the real world. But I feel confident enough to say there are about 175 to 250 shots in a 90-gram CO2 cylinder — depending on how the gun is operated.


Don’t think of this trigger as one- or two-stage. It feels like a single-stage, but there is some feedback through the blade that feels like the internal mechanism is advancing the belt. I found it surprisingly consistent and light, even though the trigger-pull gauge says it breaks at 8 lbs. It’s difficult to explain, but the curve of the trigger blade makes it seem lighter than it is.

Evaluation so far

As you can see, I am spending a lot of time with the MPX. I’m doing this because of all that’s been written by people who evaluated early guns that never should have left the factory. The gun I am testing for you should be very similar to what you would receive if you ordered one from Pyramyd Air today.

I find that the gun meets its advertised velocity specification on the nose. Read the report to understand all that entails. I find the magazine easy to load, once you acquire the knack. The manual is poor in this area, but the directions I have given here should make things clear.

The MPX has functioned 100 percent thus far. It’s reliable and does all it is supposed to do. I like the easy trigger, and the ability to shoot rapidly.