SigAir ProForce MCX Virtus AEG airsoft gun: Part 5

by Tom Gaylord
Writing as B.B. Pelletier

Virtus AGE right
SigAir ProForce MCX Virtus AEG right side.

Part 1
Part 2
Part 3
Part 4

This report covers:

  • 110 mainspring
  • BUT
  • Prediction
  • Changing the mainspring
  • Assembly
  • Performance
  • 0.20-gram BBs
  • Rock and roll
  • 0.25-gram BBs
  • Battery
  • Summary

Today we’re going to have a little fun. I know some of you would like to work on spring-piston airguns but you just don’t want to jump into the deep end of the pool — as in buying expensive tools like a mainspring compressor and parts that may or may not work as you expect. Today we are going to change the mainspring in the SigAir ProForce MCX Virtus AEG airsoft gun, and we will do it with two Allen wrenches — nothing more! This is a job any of you can do. Then we’ll test the velocity of the gun and see what impact the new spring has made.

110 mainspring

You may recall that Sig bundles a 110 spring with the Virtus, while the 120 spring comes installed in the gun. First off — what do the numbers 110 and 120 mean? That rating relates to how fast that spring will propel a 0.20-gram BB in meters per second. So a 120 spring should propel a 0.20-gram BB at 120 meters per second, which is 394 f.p.s. That’s regardless of what airsoft gun it’s in.  A 110 spring should propel the same BB at 110 meters per second, which is 361 f.p.s.


Airsoft springs are also rated with an M or an S (which can also be an SP). The M spring is the one that’s rated to toss a 0.20-gram BB as described above. The S or SP spring is rated for a 0.25-gram BB. The velocity in meters per second remains the same, but since the 0.25-gram BB is heavier, the gun will naturally be even faster with a lighter BB. So the higher the number the stronger the spring and M versus S or SP also figures in.

The 110 replacement spring that comes with the Virtus is an M110 spring, and Sig recommends using 0.20-gram BBs in the gun. They don’t say anywhere that I can see whether the 120 spring that comes installed is an M or an S, but given the ammo recommendation, I believe it is also an M120 spring.

So, what sort of velocity did we see from the 0.20-gram BBs with the 120 spring installed? Sig said to expect a 370 f.p.s. velocity, but we saw an average 410 f.p.s. speed. What I just explained was what the manufacturers say to expect from a 120 spring — 394 f.p.s. That’s real close to 410 f.p.s., so again, I think the gun had an M spring. An S120 spring would have given 394 f.p.s. with 0.25-gram BBs and probably 430 f.p.s with 0.20-gram BBs. Of course, that’s just my guess.


So, the Virtus that I’m testing shot on the fast side with its M120 spring — assuming I am correct about it being an M-rated spring. Therefore, I predict that it will also shoot on the fast side with the M110 spring. Instead of 361 f.p.s. I predict a 0.20-gram BB will average 380 f.p.s. I am writing this before shooting the first shot with the new spring.

Changing the mainspring

Changing the mainspring is very easy. First, extend the wire buttstock all the way and then remove the 3mm Allen screw on the right side, where the stock meets the receiver, and the entire stock slips up and off the receiver. By the way, the Virtus manual says the screw is 8mm, but it’s actually 3mm — no doubt a mistake in transcription. When the screw is out, a plastic keeper that it passes through also comes out and the stock slips up and off the rear of the receiver. When reinstalling the stock make sure the V-notches on both sides of the receiver line up with the two heavy wires in the stock.

Virtus AEG stock off
With the screw and keeper out of the stock the entire  assembly slips up and off the receiver.

Once the buttstock assembly is off the gun, the rear of the spring guide is exposed. The manual calls it a screw that you turn 180 degrees, but it’s actually a bayonet keeper. Turning 180 degrees aligns the flanges of the keeper with their raceways and the mainspring pushes the keeper out. Remember that the keeper is under spring pressure, so pressing in on the wrench helps loosen it for turning.

Virtus AEG receiver
With the stock off the rear of the spring guide (arrow) is exposed. Insert a 5mm Allen wrench and turn the guide counter-clockwise 180 degrees.

Virtus AEG  spring out
When the bayonet lugs align, the spring guide is free to come out. This is how far the 120 spring pushes the guide out. You can restrain it easily with your hand.

The two springs compare in this way. The 120 spring is made from heavier wire and the 110 spring is longer — though that may change after a few weeks in the gun. Both springs are wound with what the airsoft industry calls irregular pitch, which means some coils are closer than others. That allows the spring to start compressing easier and then increase in tension the more it’s compressed. It’s supposed to be easier on gearboxes, though you will find a lot of arguments on both side of that issue!

Virtus springs
The softer M110 spring is on top and the 120 is below. It’s not easy to see, but the 120 spring is made from heavier wire. Both springs are wound with an irregular pitch.

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The Virtus goes back together the reverse of the way it came apart. And it’s just as easy as it sounds. It’s taken me a long time to describe a process that took me 20 minutes to perform — again with just two Allen wrenches.


Now, let’s find out what installing this lighter spring has done for us.

0.20-gram BBs

First to be tested were 0.20-gram BBs. I believe I am out of the BBs Sig sent with the gun so I used 0.20-gram TSD competition BBs. The average velocity for 10 was 380 f.p.s. Sometimes old BB gets it right on the nose!

The spread went from a low of 370 to a high of 383 f.p.s., so a 13 f.p.s. difference. With the 120 spring the average was 410 f.p.s. with a 6 f.p.s. spread.

Rock and roll

I emptied the magazine on full auto and truthfully could not tell any difference in the cyclic rate this time versus with the 120 spring. There may be some but it’s pretty small.

0.25-gram BBs

Next I tried the same Open Blaster 0.25-gram BBs that I shot before with the heavier spring installed. The average this time was 343 f.p.s. with a 5 f.p.s. spread from 340 to 345 f.p.s. With the 120 spring the average was 365 f.p.s. with a 2 f.p.s. velocity spread. At the end I dumped the magazine on full-auto again, remembering to fire a couple shots on semi-auto afterwards to relax the spring.

I did not load heavier BBs for testing. I think the 0.25-gram BBs are as heavy as I would go with this spring, given the velocity we have seen.

So, the 110 spring varies in velocity slightly more than the 120. Of course this spring is brand new and may settle down a bit after a few thousand rounds have been fired.


I would like to point out that the battery has never been recharged since I started the test and it is still going strong. Not only has it fired many hundreds of rounds including lots of blank shots, it has also been stored charged for two months.


Next we test the gun for accuracy. If the accuracy is reasonably equivalent to the 120 spring I think I will leave the 110 spring installed. It is no doubt a little easier on the gearbox.

This Virtus AEG is a serious airsoft gun, as I have maintained all along. This is the kind of equipment a skirmisher wants to have for close-quarters battle!

42 thoughts on “SigAir ProForce MCX Virtus AEG airsoft gun: Part 5”

  1. B.B.,

    Although not deep into the innards of these things, I appreciate how this relates to spring piston power plants.


    PS: Section Rock and roll Second sentence: “There may (be) some but it’s pretty small.”

  2. BB,

    I do apologize for this high jacking.


    That was one of the issues I had with the plastic stocks of the Aspen and the Liberty. The Avenger looks much better. I had a Gamo CFX with a plastic stock. That hollow “sound box” right next to your ear made each shot pretty loud. I too filled the voids with foam rubber. It helped immensely. If one of these was to come live at RRHFWA, I am certain the voids would be filled first thing.


    The Dark Side is not that bad really. You can get into PCPs for less than you might think. If you go with .177, most of the time a good quality hand pump is all you are going to need. If you go for those air hog big bores though… As for the Avenger, it can be tuned to be hand pump friendly. Stephen Archer is doing an extensive review/test with one.


    You do have to take it easy on the wife though. Toys are great, but she needs to feel that she takes precedence.

  3. B.B.

    If “irregular” pitch springs reduce the cocking force, or at least the perceived cocking force, why don’t full power spring guns use them? Do you know what other popular uses of these irregular pitch springs are?

    24 little hours…


  4. Hi B.B.

    Another great post B.B., and I am happy to hear that you were able to perform the swap of mainspring with only an easy available Allen tool, but for most of efforts you need a special tool you have to buy or make to get the job done. I have an ASG Dan Wesson pellet revolver for about a year when the threaded collar that centers the tip of the Co2 cartridge over the Co2 puncture port just fell off when removing a spent Co2 cartridge. It looks like the tool required would be a flat blade screw driver with the center of the blade filed down so as to clear the puncture port, but where do you go to buy such a thing as well as any spare threaded collars which seem to match those in use by UMAREX? I have tried contacting ASG and Pyramyd Air for suggested repair sources but the current medical crisis must be slowing the response. Other than making such a tool or contacting other air gun supplies for parts do you have a suggestion? I do not know if even more is involved as the the threads in the part that mates with this threaded collar may also be in need of repair as well, I hope not but that is a worse case situation. Might make for a useful post for all of us when you have the time. Thank-you.

  5. Looks like a well made firing replica, which is not a cheap thing.
    Tool making is the highest form of making, or at least it was. When I get mad because my .dll files are corrupted,
    I turn tale and run to the garage and fix that doohickey.

  6. Repairing airguns.
    Lets face it, airguns were not intended to be repaired by customers as are a lot of other products. Most ‘low cost’ airguns are considered disposable when they break.
    Specialized assembly tooling, jigs and such, and processes probably play a major part in this. The reason we have the likes of gunsmiths and other repair specialists. Also, it is probably not cost effective to send it back for repair.
    Some may be easier to work on by design but not by the general public.

    A lot of mechanically inclined airgunners with tools and an ability to improvise and overcome the obstacles preventing an easy repair or mod have no problem jumping into them. But, they too will frequently rely on specialists for parts such as ‘O’ rings and parts kits.
    Support from the marketing company is a major consideration for purchasing if you plan to make a hobby out of airgunning, but, think about the problems that come up when the airgun is made in another country. They most likely will not have the ability to repair them or have parts on hand for replacement.
    One company I investigated before I purchased a new airgun honestly admitted that, for now, they currently relied on returned airguns as a source of replacement parts. They also said they would do what was necessary to repair them to honor the warrantee. They happened to be an established firearms sales company and recognized the importance of customer support.
    A lot of the people on this blog are “Tinkerers” and enjoy the challenge of repairs and mods and are always willing to help if they can but sometimes you simply have to improvise if you wish to avoid the hassle of trying to get an airgun repaired that’s out of warrantee. Something to consider before you purchase.

    • Bob M,

      These are indeed important considerations to take when buying anything, not just airguns.

      The airgun world is still a relatively small market, but it is growing. Customer support is important, even for us “Tinkerers”. More and more of the newer ones are not designed to be easily worked on. This is one of the reasons I personally prefer the “old gals”. They were made with steel and walnut and are very durable and fixable. I do have a few “new” airguns, but they too are usually robust and of good quality.

      Like everything else in this world, if it sounds to good to be true, it probably is.

  7. All,

    I have already told BB. Yesterday afternoon my son-in-law died suddenly. He was a budding airgunner who very much enjoyed the Diana 46E I gave him and going with me to airgun shows and Fun Shoots at Lloyd’s where he had the opportunity to try out all kinds of airguns. He was crazy in love with our daughter and grandson and a devote Christian. He was 36. He will be missed here, but we shall see him soon.

    • RR,

      I am sorry for your loss. (I actually hate saying that) as it is the people that are left to pick up the pieces. Prayers for you and them on doing just that. Can’t say no more than that.


    • RidgeRunner,

      An apparently untimely and early loss is always difficult to comprehend. A great Son-in-law is a rarity and one you want to go shooting with is even more so! My heartfelt condolences to all your family on this grievous loss. The Good Lord’s infinite wisdom is sometimes hard to comprehend!

      Grandpa will need to step up for a time! I pray for you to find the strength and wisdom to do the task well.
      Grandpas do in fact matter!


    • RR
      When I was a kid there seemed to be a lot of men with limps, walking around with canes or in wheelchairs and funerals were very common. I can remember my father crying in his beer, “All my friends are dying off”. He made it to 58.
      It wasn’t till I was facing the draft into the military that I realized they were all Veterans of WWII. Wounded and most likely suffering from PTSD as we know it today. Everybody smoked and drank after work and just about every weekend. Bar parties were common place. And you can believe there was a lot of comradery among those surviving vets with cause to celebrate for the rest of their lives.

      Today we all expect to live a long life with all the advances in medicine and healthy life styles. Early deaths are unexpected and devastating today.
      I am sad for your family but I am sure with Gods blessings you will shine in your daughters eyes and become a hero to your grandson. I hope the rewards will far out way the demands and everyone will recover soon.
      Bob M

    • RR,
      Just read about the loss of your son-in-law! My sincere condolences to you and the family in this most difficult time. It always seems worse when the person is young, and the passing is totally unexpected. It’s a traumatic shock to say the least. Things like this are far beyond our understanding. May our heavenly Father comfort you and your daughter and give you peace. :'(

  8. A bit of trivia for the youngest among us about progressive springs.
    Before the invention of steel belted car tires we had soft bias ply tires (high profile) that were very good at absorbing road irregularities. Steel belted tires were great for performance because they kept the tire thread relatively flat on the road where as bias ply tires had a tendency to roll over on to the side walls of the tire and lose traction in a high speed turn.
    It wasn’t long before they discovered that steel belted tires were very bad at absorbing road irregularities so they came up with progressive springs to compensate for them. They easily absorbed the bumps with a softer spring rate at first and got stronger as needed to keep the suspension from bottoming out.

    Airsoft springs are compressed by an electric motor, for the most part, over and over again for a full auto or semi and work better if the spring is easily compressed at first. Piston momentum or inertia assists in compressing the spring and makes it easier on the motor. Similar to a hand pump in motion.

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