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Improving the accuracy of the SIG MPX pellet rifle

Before we begin, yesterday marked the 80th anniversary of D-Day — the start of Operation Overlord to liberate Europe. Most of our veterans have passed on, but we are able to enjoy our freedoms because of what they did. We remembered those who didn’t return on Memorial Day. Today we remember those who did come home.

Today reader Ian McKee, whose blog handle is 45Bravo, tells us about refining the pellet belt inside the magazine of the Sig MPX repeater.

If you’d like to write a guest post for this blog, please email me at blogger@pyramydair.com.

Take it away, Ian

Improving the accuracy of the SIG MPX pellet rifle
by Ian McKee

Sig MPX.

This report covers:

  • Report history
  • More belts
  • Mark chambers
  • The test
  • Benjamin Single die pellets
  • JTS wadcutter pellets
  • Summary

BB reviewed the SIG MPX back in February 2016. The accuracy was acceptable for plinking at 10 yards, in the 1.5 inch (38.1mm) range for most pellets he tested.

I have been shooting the Sig MPX for the last few months. As BB said in his review the trigger isn’t bad. It’s about like a Mil-Spec AR 15 trigger with only a little longer travel in stage one.

This gun is similar in POU (Philosophy Of Use) to the Crosman DPMS SBR (Short Barrel Rifle) line of CO2-powered airguns. They are meant to replicate the firearm version of a modern sporting firearm in size, weight and ergonomics. They are great for engaging multiple action targets at reasonable ranges. Or protecting the back yard from the onslaught of feral soda cans. 

As I was shooting the gun at longer ranges out to 25 yards, I started noticing that certain pellets wanted to group in certain areas of the target. With an average overall group size in the 3-5 inch (76mm – 127mm) range over the full mag of 30 shots.

While pondering this I came to the realization that the belt fed magazine was like a regular PCP magazine. Meaning each chamber that holds a pellet is different and they are probably affecting the accuracy. 

More belts

I needed more belts to test my theory that was brewing in the back of my brain. I had picked up the SIG MPX used so I only had one 30 round belt. A quick search of Pyramyd AIR showed they carry extra magazines and belts for a reasonable cost. You get one mag, and three of the 30 round belts. 

I have found that the belts are able to be “delinked” and reassembled in a different order. 

Mark chambers

I started marking the chambers that hold the pellets with an indicator (a black permanent marker) as to where that particular chamber  tended to group on the target. 

That involved taking a shot and seeing where it hit on the target. Then removing the magazine and making a mark on that chamber as to the direction the pellet landed in relation to the point of aim. 

The links are molded in sets of two chambers. When marking the chambers while looking at the mag from the rear, put a dot around the rear of the chamber showing the direct in the pellet impacted the target from the point of aim. Insert the mag, fire another shot and remove the mag and repeat. 

MPX belt markings
Mark the outside edges while the belt is in the magazine after each shot. 

One belt is shown outside of the mag to give you an idea of its orientation and link positions.

Each link consists of two chambers. The first link indexes with the bore when the tab is just before top dead center. The second chamber of the link indexes when the tab is just past top dead center. 

MPX different POI
These two links do not shoot the same. The link on the right will have both pellets hit to the same general point of aim. The link on the left does not, the second shot of that link will hit high left. 

I did this three times per belt. To get a better idea of the clusters on the target, there are 90 shots per belt, for 360 shots total as I have four belts. 

The belts just pop apart, and you arrange the belt segments that all have the same tendency to group in the same general area. While not completely curing the issue, it has greatly improved the groups and seems to have removed a lot of the uncalled fliers. 

I now have one very accurate belt, and two belts that shoot ok, and one made up of the leftover links that just didn’t match up. 

The test

I also decided to test two new pellets that were not available when the SIG MPX was introduced back in 2016.

Instead of a red dot, I mounted a UTG Accushot 4-12×44 for the test to have the best chance of reducing aiming errors. 

Benjamin Single die pellets

First up, the new Benjamin Single Die .177 domes weighing 10.5 grains. 30 shots went into 1.49-inches (37.8mm) at 25 yards from a rest. If you were to take out the outlying hits, about 22 of them went into an inch!

MPX Bullseye
Thirty Benjamin Single Die pellets into 1.49-inches at 25 yards!

The high velocity was 349fps , the low was 320fps, with an average of 333fps. With only 2.4 ft. lbs. of energy at the muzzle, this is a plinker, not a hunter.

Build a Custom Airgun

JTS wadcutter pellets

Next up was the JTS 7.87 grain wadcutter. Using the same belt and adjusting the scope before starting. 30 of the JTS pellets went into 1.44 -inches (36.5mm) at 25 yards. 

MPX JTS wadcutter
Thirty shots into 1.44-inches I think we found an airgun that likes the JTS wadcutters. 

With a high of 427 .fp.s., a low of 420 f.p.s. and an average of 410 f.p.s. and only 3.7 ft. lbs. of energy, this airgun is an excellent backyard tin can roller. 


Here we are eight years later, the SIG MPX pellet rifle is still being sold. It is available in CO2, or it can be used with a 13 cubic inch precharged and regulated air tank that can be chosen at the time of purchase. 

The gun I tested today was using the disposable 88 gram CO2 bottles

The MPX still fulfills its intended purpose,  a fun ”semiauto” plinker that also replicates the actual firearm in almost every way including the weight. 

By reorganizing the links in your belts you can improve the accuracy and improve its hit ratio at longer distances. 

Shoot safe, have FUN!


author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

40 thoughts on “Improving the accuracy of the SIG MPX pellet rifle”

  1. B.B.

    While we have D-Day in our thoughts, please remember that more Allied solders were killed on June 7th than on June 6th.

    May they all rest in peace, we owe them our freedoms,


    • Yogi,

      I goofed! That intro belonged on the 6th. So I changed a few words to make it work for today.

      I wrote that intro on Wednesday to get prepared for a different function on Thursday and I goofed.


    • Agree it does fit well in the Airgun world.

      I can not take credit for it though, one of the Youtube gun guys coined the term about 10 years ago.

      For some reason it has never gained traction as terminology in the industry.


  2. Ian,

    This is a personal issue I have had with all Sig CO2 rifles and pistols. I for one have never been a fan of each link position being the actual chamber. That issue has had me rejecting even the thought of owning one of these. This is a brilliant solution to one of the issues that I have had with theses airguns over the years.

    I would not want to be a feral soda can around your home.

    • We see it in many other airguns as well, not just the SIG belt guns.

      The Crosman 1077 uses a similar way of holding the pellet in its clip (little small ridges running the length of the pellet chamber.

      And in the individual cartridges used by the Umarex revolvers and lever guns.

      With the cartridge guns you really can not control the orientation of the chamber as it is loaded into the firing position.

      With the belt arrangement, it is mechanically indexed into almost the same position every time.

      With one of the rifles BB is testing he has a bad chamber in the #4 position.
      But the rest group ok.

      He can either skip loading number 4, or just blow that shot off the paper.
      Or TRY to true up that chamber.
      If that fails, toss the mag.

      At least with these, you can swap around until you are satisfied with the results.


      • Ian

        I thought I was nuts for running nearly 60 different pellets through all my new guns to find the most accurate, but you take the cake, buddy! LOL

        I did try a similar test with some of the disk shaped mags for Colt Python revolvers that I own, but they were BB versions and I never found a correlation between the “cylinder” hole and the POI of the BB. I’m guessing that it was because initial alignment inside the hole wasn’t an issue with a sphere, as it could be with a skirted pellet.

        If I understand how the links join each other, then I have to say that I’m more than a little surprised that variations in the outer rims of the links and the inner surfaces of the metal “couplers?” didn’t introduce even more variability. Personally, I would have been so convinced of that preconception that it would have keep me from even pursuing the experiment because of it likely being a waste of my time in the end. I’m so very glad your convictions, otherwise, lead you to do this and share it with us.


      • Ian,

        Being one who almost exclusively shoots single shot airguns, I rarely experience such issues. The orientation of a particular projectile in the chamber can affect how it exits, but just how anal do you want to get?

        Some time back, BB experimented with single loading by orientation of his AR platform. Though the results were positive, I was not truly impressed by such. IMMHO he would have done better experimenting with varying the loads, different barrels or admitting the AR platform is really not that great.

            • How anal do you want to be? That’s up to the individual.

              While I have never shot a Schuetzen match, I have attended one.

              They use a single cartridge for the entire match, the cartridge has index marks to match the chamber so it is in the same orientation every time.

              They used a special loading tool to ensure the projectile was loaded to the same depth with every shot.

              The cartridge was never resized, just deprimed, re primed, and meticulously loaded with a new charge, and placed into the chamber with the witness marks all aligned.

              Did it help?
              I don’t know, but it was interesting to watch the procedure, and each shooter’s approach to eek the utmost accuracy out of their rifle.

              If I remember, it was 30 minutes for a couple of sighters, and 10 shots for score.


              • Ian,

                LOL! They enjoyed doing such! Their joy was not in the winning, but in the procedure used to compete. They could have cared less if they won or not.

                My father and I used to reload. We would spend days reloading ammunition for his .25-06. The results were that we could shoot one-inch groups at three hundred yards.

                I once attended a shooting match where “the judges” spent almost one hour trying to determine which of two shooters had the smallest five shot, one-hundred-yard group. Both shooters were using .30 caliber rifles. The judges were using a .30 measuring device to try and determine which shooter had made the larger hole. Five shots. One hole. One hundred yards. That is anal.

  3. Well done Ian. I had a Sig MCX but passed it on to a young man who’s family is friends with ours. He had aspirations of being a Highway Patrolman. One of the reasons that I chose that gun for him was that it fit well with his career path for practice and choice in firearms, also because I am an accuracy nut and it just didn’t cut it, so it hardly ever got used. I will pass this blog on to him to do as he pleases with it as I had 2 spare mags and extra belts with it.

    Oh and the young man just got accepted to the police academy to become a Highway Patrolman. Proud of the kid.


    • Very cool Bob, I wish him well and a safe career.

      I could not imagine being on the force with the way people treat Law Enforcement these days.

      It takes a certain attitude and disposition to be a street cop dealing with the general public daily.

      Some people have it, some don’t.


  4. 45B

    The vast majority of police officers are honest hard working people who chose the profession in order to help people. It is unfortunate that we only hear about those few who are not.

    It seems that the ubiquitous cell phone has led all too many to use the cameras on them to capture images of some officers at their worst. Or, at least, those are the only ones they put on TicToc.

    Of those that I know personally and those I have had the misfortune of meeting while they were doing their jobs,, I can only say that they have always been professional and courteous to me. In a few cases, friendlier than I probably deserved.

    Their jobs are certainly difficult,, and too often dangerous. I should imagine that the stress involved would be one of the major reasons so many leave the profession.

    I have great respect for those who maintain their ethics and morality in the face of the hurdles they face daily. I also have sympathy for those unable to do so. I don’t think I would be able to live up to the standards the public forces on them, daily.

    As in all professions,, and life in general,, there are a few who simply,, don’t play well with others. Sorting them out is difficult.


  5. Interesting Ian!

    You proved again that in the pursuit of accuracy attention to all the small details will add up to a major improvement.

    Not a surprise that the belt and magazine fed repeaters are subject to accuracy issues because of misalignment. I’m in the habit of inspecting/polishing every magazine before use (doesn’t everyone?) and when testing/tuning single-load the projectiles.

    Guess it doesn’t matter for casual plinking but it definitely does in the pursuit of sub-MOA groups at extended ranges.

    Thanks for the blog!


    • Hank, the only time I test and tunes mags are when they break and have to be repaired, when the springs fail.

      Other than that, not really,

      Before the Pyramyd AIR cup last year, I only used one factory mag, the rules required that you use only 10 rounds in the first mag.

      The next mag could have as many as you wished.

      I had 1 factory mag that worked well, then two aftermarket high capacity mags, and then a fourth factory mag as an absolute last resort. That I never had to use.

      Normally I am single loading for bench work, and accuracy.

      I think we need a guest blog about how you tune and polish a defective mag.


      • Hank,

        I second Ian’s request. I have no idea of how to align the chambers in a rotary magazine. I’d like to know what you do!


        • I seem to remember GunFun1 commenting about just that. Might have been a year ago,, or possibly three. Truthfully, I don’t remember what the blog was about that elicited the comment. But as we all know, often the comments run far afield of the blog topic.


  6. 45Bravo,

    Great report and a great deal of work went into making this SIG shoot way better than stock. I can only imagine the MSRP (Manufacturer Suggested Retail Price) your improvement work would require for even a minimal level profitability; maybe US $175.00 per accurized magazine.
    Cheap airgunners should learn that lesson!

    Great Guest Blog at many levels!

    Now for som PICKY shootski observations:
    In the second image in: Mark chambers
    I’m confused by the caption. I would have reversed the explanation based on the visual:
    “These two links do not shoot the same. The link on the left will have both pellets hit to the same general point of aim. The link on the right does not, the second shot of that link will hit high left.”
    That means i probably don’t understand your marking system.
    In the JTS Wadcutter test i think you may have a typo:
    “With a high of 427 .fp.s., a low of 420 f.p.s. and an average of 410 f.p.s. and only 3.7 ft. lbs. of energy, this airgun is an excellent backyard tin can roller.”
    I don’t know how to get a geometric or arithmetic average that falls outside of the maximum and minimum values.

    Even after those it is still a BZ (BRAVO ZULU) hoist worthy Guest Blog!

    Thank you,


    • As for the photo of the two links, the link chamber is at top dead center of the mag when fired, then it is advanced when the trigger is squeezed for the next shot. .

      You pull the mag after each shot, and put a dot around the chamber in the direction the shot landed.

      The link on the left the two shots on the left both land at the 12 o’clock position.

      The link on the right, the pellet in the first chamber lands at 12 o’clock, the pellet in the second chamber lands between 9 &10 o’clock.


  7. BB, Thank you for mentioning D-Day. Whenever I’m out, and I see a veteran, I always take the time to go over and shake their hand and thank them for their service to our country; I’ve even had the privilege of doing so for some WWII veterans…God bless them all!

  8. 45Bravo,

    Ian i think i broke your code.
    The key is to think: “…when the tab is just before top dead center. The second chamber of the link indexes when the tab is just past top dead center.” simple.
    The first part of your explanation was a Distractor for me; reminded me of the GREs (Graduate Record Exams) i hated those smug test writers! Did this one to myself.


  9. 45Bravo,
    Nice bit of troubleshooting and repair. Thanks for sharing it.
    A simple way of making the most out of your airgun and now another Round-Toit is needed for me.

    Now for all those spring-loaded round mags, simple. Create a logbook for each mag if you really want to get the most out of each. Assign the hour of a clock and some form of indication of how far away from center it is to each mag hole position. Mark the mag for ID and create an index for each airgun it can be used in.

    For Full-Auto Operation you might need to take a speed-reading course.

    • Bob M.
      I am Dyslexic, I definitely don’t read in full auto mode…

      I do have a tendency to speak at full auto speeds, punctuated with pauses while my mind manually cycles the action to clear a jam.


  10. 45Bravo,
    I am just the opposite sometimes; my brain operates faster than my mouth manipulation muscles and words turn into gibberish, running into each other.

    Looks like there is a hard choice out there in the airgun world. A Fortify for $55. or the Marshal for $70. You get a copy of John Waynes signature for the $15. increase. I wouldn’t go looking for his DNA on it.

    Now, my now discontinued 1077 Wood Stock might temp someone. “From my cold dead hands” Charlton Heston is engraved on it. It was obviously owned by Moses, a plausible conclusion? 🙂

  11. Shootski,
    A good friend of mine, a retired Navy Senior Chief Petty Officer sent me a picture of himself along with his aircrew standing in front of his Aircraft, but it looked like the hull of a boat? News to me.

    It was Nov 1966, VP50, at Sangley Pt, in the Philippines. Vietnam era. Coincidently I was in the Navy a week or so later. Looks like it was a SP-5B or PBM-5 Mariner? Seaplane. ‘T’ Tail
    Their nickname was the Blue Dragons and I’ll assume that was when they had the PBY-5A/6A Catalina. It looks gray to white in the old faded black and white picture. He always talked about flying in a P-3.

    Can’t imagine what it would have been like landing in water. Might be a little hard to reach the engines with a ladder when it’s floating.

  12. Bob M,

    Senior Chief would have been bobbing around in a P5M Marlin in 1966 the last active USN Flying Boat. It was to be replaced by the P6M Seamaster which would have been the first turbojet propelled Flying Boat; but that was cancelled.
    “Might be a little hard to reach the engines with a ladder when it’s floating.” They used bosun’s chair (or boatswain’s chair) to (rarely done) work on engines while floating.
    The P-3 Orion (Lockheed Electra) and variants were an interesting platform to fly given their responsive-massive amount of power, and Transsonic airframe. The P-3 was one of the best of the large Turboprop aircraft that will ever be built.


  13. B.B. and Readership,


    So what exactly is .30 caliber? What is .303 calibre (British round British spelling) for added confusion? What about .308 caliber?
    Unfortunately none of them turn out to be what most airgunners and many firearm shooters believe. If you do a web search be prepared for MASSIVE confusion and a great deal of bad information.
    In discussion with Jerry Cupples about a .308 caliber SlugGage he reminded me that he sells a .30 caliber PelletGage i have one. I tried the bullets (slugs) i shoot in my .308 DAQ and they didn’t fit, as i suspected, even in the largest (7.70mm) aperture. Measuring both the bore as well as the bullets that shoot well turned out to be mostly 7.82 – 7.83mm (≈ 0.30788 – 0.30825 inches) in range.
    Slug tins are mostly labeled as .30 caliber/7.62mm and they fall through the .308 DAQ bore when the muzzle is pointed downward. They “group” like a shotgun has been used to make them; this inaccuracy was not unexpected both because they go out supersonic even sideways and how light they are :^)
    So the purpose of this post is twofold:
    First to give everyone a heads up that .30 caliber on a tin/box may not mean what you think. Unfortunately it also means that .30 caliber/calibre airgun you bought might not be 7.62 or anything close.
    The second purpose is to find a way of knowing how many folks would want a .308 caliber SlugGage or even a .303 calibre SlugGage.
    Or even just finding a way to know how many are wanted or needed.
    Your ideas are welcomed.


  14. It would be interesting to chronograph pellets fired from the various links and maybe shoot them into a tub of water to observe if the less accurate ones are being deformed when fired. My experience with a Gamo Swarm was that the auto loading mechanism was clipping some of the pellets and causing low velocities.

    It makes me suspect that something similar is occurring with the Sig belts. There’s got to be some underlying cause for the difference in accuracy you are seeing with different links.

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