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.

BUT

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.

Prediction

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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Assembly

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.

Performance

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.

Battery

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.

Summary

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!