How airsoft and BB gun magazines work

by Tom Gaylord
Writing as B.B. Pelletier

Today’s report is another guest blog from reader Ian McKee who writes as 45 Bravo. Today he tells us how airsoft and BB magazines work.

If you’d like to write a guest post for this blog, please email me at: [email protected].

Okay — take it away 45Bravo!

Ian McKee
Writing as 45Bravo

How Airsoft and detachable BB gun magazines work

This report covers:

  • It’s a replica thing
  • Low/mid capacity
  • Most of them are very similar
  • Magazine capacities
  • High capacity
  • It’s high capacity clockwork!
  • Summary
  • Coming in the future

It’s a replica thing

A lot of replica air guns have removable magazines to replicate the look and function of the actual firearm they are copied from. 

magazine lineup
From left to right, is a real 5.56/.223 30 round magazine, a 70 round mid-capacity mag, a “20 round” mag that actually holds 150 rounds of airsoft ammo, a 300 round mag, and a 850 round “fatmag”.

Both airsoft and BB guns, use the same magazine design. The only difference being the size of the projectile being used.

As much we would hate to admit it, many of our replica BB guns were first produced as airsoft guns. A LOT more replica airsoft guns are sold, compared to the replica BB gun market, partly because of laws in many countries that have more stringent regulations on BB guns than they do on airsoft. And parents perceive that the plastic airsoft projectile is less dangerous than steel BBs.  

Low/mid capacity

Spring-powered models that have to be manually cocked for every shot, normally have a full sized magazine that holds just 15-30 of the airsoft projectiles in a single column stack, and the magazine sometimes has small metal inserts inside to give the magazine some extra weight. 

low cap mags
Here are two low-capacity airsoft magazines.

Some CO2 BB guns like the Umarex Legends PPK/S, have a small removable stick magazine and keep the CO2 inside the gun, while some designs like the Sig We The People 1911 BB pistol, and the Crosman SBR (short barreled rifle) have a removable full size magazine that holds the CO2 and the BBs together in one removable unit. 

SBR mag
The magazine for Crosman’s short-barreled rifle (SBR).

Most of them are very similar

But no matter how the airguns function, the BB feed mechanism in the magazine is the same, the projectiles are held in a channel, either single stack, or double stacked, and have a spring-driven follower that pushes the projectiles up to the feed lips, ready to be fed into the chamber when needed.

1911 magazines
Do they look similar? The one on the left is from the Sig We The People BB pistol, the other one is from an airsoft 1911 I have used for over 10 years.

1911 magazines detail
This detail shot shows the similarities much better.

Magazine capacities

Airsofters that are into military simulation games (Mil-Sim), like to use a magazine that features a similar capacity as the firearm it replicates. That way the number of their magazine exchanges  plus their load out (the number of magazines they must carry) matches their real world counterparts.

High capacity

Some magazines for automatic electric guns (AEG) use what is called a mid-cap mag, where the channel that holds the projectiles under spring pressure is longer, and may hold 60-100 projectiles. One of the advantages of a mid-cap magazine is since the BBs are under constant spring pressure, they don’t rattle when you run.

midcap and hi-cap mags
There is a spring loaded tab at the top of each magazine to keep the BBs in place until the magazine is loaded into the gun. The mid-cap mag. is on the left.

midcap inside
The BBs are under constant spring pressure while inside the mid-capacity magazine.

The most common type of magazine for the electric guns are high capacity or “high-cap” magazines. They can hold anywhere from 150 to 1000 rounds or more, depending on the size of the reservoir that holds the projectiles before feeding them into the feed channel. 

magazine bottoms
From left to right, is a “20 round” mag that actually holds 150 rounds of airsoft ammo, a 300 round mag, and a 850 round “fatmag”.

It’s high capacity clockwork!   

The high -cap magazines have a hopper that you pour the airsoft BBs into, and normally they have a wheel on the bottom of the magazine that you wind to compress a clockwork style spring that drives the feed mechanism to feed the BBs from the hopper to the gun. 

highcap mag wheel
That toothed wheel (arrow) is wound to compress a spring that pushes the loose BBs up toward the feed lips of the magazine.

As you wind the magazine wheel on the bottom, you hear clicks that are the anti reversing mechanism that keeps the spring from unwinding.  When the magazine is fully wound, a clutch in the winding assembly causes the wheel to “slip” and you hear the clicking sound different. The magazine is fully wound at that point, and you can normally empty the contents of the magazine without winding more. 

Unlike the other magazines that may have the gas stored in them, the high-cap magazines just hold and feed the projectiles. As the gun cycles, it strips off the top projectile, feeding it into the chamber. It doesn’t matter if the gun is electrically operated, or run from compressed air, or CO2 from a remote tank.

mag wheel in magazine
The magazine wheel in the magazine.

high cap mag insides
The insides are all the same, the larger magazine shells just hold more of the projectiles.

As you can see, they all have the same basic components. The highcap magazines hold the projectiles in the open areas and feed them into the feeding channel, and then into the hopup/nozzle area of the gun, as they are fired.

hi-cap mag feeding
Here you see how the hi-cap mag. takes the loose airsoft BBs and organizes them into a feed channel. That spring you wound with the exposed wheel powers this internal mechanism.

The major downside of a high capacity magazine, if you are playing in an airsoft game, is once you start shooting, there is open space in the hopper area of the magazine, and the bbs rattle when you run. So when you run from cover to cover, you sound like you have a half empty container of Tic Tacs in your pocket.   

By these photos, you would think all airsoft guns are based on the AR-15/M4 platform, but you would be wrong. These were just the magazines I had on hand. Magazines of all capacities can be had for any replica, AK47 & AK74, FN-p90, HKMP5, FNFAL, HKG3, MP7, Thompson M1A1, M14 (ALL of these replica machine guns, and others not listed do come with the giggle switch here in America.)


If you can think it, it has probably been made into an airsoft gun, with magazines and gearboxes adapted to feed it and fire it. 

Airsoft replicas are available from a Mosin Nagant 91/30, to an electrically operated M134 multi barreled minigun!

M134 minigun
Here is a $3,840 M134 airsoft minigun!

Coming in the future

In a future article, we will cover airsoft gearbox types, and other types of airsoft power plants. 



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.


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!