Dissecting the Sig Sauer Mosquito .22 Pistol

Dealing with the pesky insect it’s named for could be easier than dealing with the Sig Sauer Mosquito. You can buy the tools you need to control the flying pest; for the pistol, you can’t. I won’t venture to guess what thoughts dominated the Sig Sauer design team when they created this bug on their drawing boards and computers. It’s possible their goal was to make life difficult for every gunsmith destined to deal with it. At least, that’s the way it seems tome.

Now that those comments are off my chest, I must admit the Mosquito is a skillfully built pistol. It balances well. It’s sturdy. It looks business-like and the factory sights are more than acceptable. That said, I’d never recommend the Mosquito to anyone considering the purchase of a .22 pistol for serious target shooting. A precision bull’s-eye gun it isn’t, and it’s a bit pricey for a plinker. The Mosquito is usually purchased as a Standard version.

It can be converted to a Sport or Silencer version by removing the Standard barrel and replacing it with one threaded at the muzzle end. To those threads can be fastened a weight for the Sport model or a silencer for the Silencer version. Since these devices are illegal for civilian use, I’m chalking up that option to over-engineering and/or simple ignorance of our laws. Who really needs a silencer on a .22 anyway? Except maybe a Mafia hit man. The version shown and discussed here is the Standard.

Though insignificant differences in specifications exist between it and its brethren, one Mosquito is mechanically the same as any other. All specifications stated here are for the Standard. The caliber is .22 Long Rifle, the weight without magazine is 22.9 ounces, and the out-of the-box trigger pull is about 4 pounds in single action and a little over 12 pounds in DA mode. Magazine capacity is 10 rounds. There is both a manual safety and a decocking lever, the latter being a rare feature on a .22-caliber semiauto.

There is also a key-operated integral safety lock that totally freezes up the gun when engaged. When the magazine is completely out of the gun it cannot be fired. There are some minor details concerning the basic 22-caliber Mosquito Standard Model pistol is shown here. With the proper recoil spring installed, it can digest both standard and high-velocity ammunition. servicing and operation in the Mosquito’s owner’s manual. Though minor, I’d consider them important.

Owners make a habit of losing their manuals, of course, but y calling SIG Arms Field Stripping Make the gun safe and hide your eyes behind something transparent that will protect them. Remove the maga-

zine. Fully retract the slide (#4), lock it back with the slide-catch lever (#18), and keep your fingers out of the ejection port. (I’ve had mine painfully betrayed many times by a slide that disengaged from a catch all by itself, and slammed forward too rapidly for me to perform a defensive retreat.) Now depress the catch lever to close the slide. Rotate the takedown lever (#16) clockwise until it points toward the muzzle.

Retract the slide fully, lift it at the rear to clear the frame, and control its movement forward. Remove the entire slide assembly to the front, and then pull the recoil spring (#3) and guide (#2) off the barrel. That takes care of that. Detailed Disassembly I’ll begin in my usual manner, with the slide. Undo the rear-sight screw (#68) as far as you can without forcing it out completely. Remove the right lever screw (#67) with an Allen wrench.

Choose a punch appropriate for the diameter of the slide breechblock pin (#7) and use it to push the left lever (#46), its spring (#30), and detent ball (#31) out right to left as a unit. The detent ball is only .02 mm in diameter. The spring isn’t a heck of a lot bigger, and they’re both housed inside the left lever. Take precautions against losing them or be prepared for a lengthy search.

Move the rear sight from left to right until it releases the breech-block assembly (#9), then tilt the slide forward to remove the block. The rear sight can be taken off the slide now, but I see no need for it. At this time, there are no aftermarket or alternate factory sights made for the Mosquito. I’ll bet the schematic does to you exactly what it did to me. The firing pin (#11) and spring (#12) appear to be housed in the barrel breech. They aren’t.

You’ll discover, as I did, both pin and spring are in the breech block and retained by a left-to-right-driven pin that’s not shown on the drawing. Combined with that pin in securing the firing pin and spring are the safety lock (#13) and its pin (#14). Both the Then depress part #66, remove #16 and #58, and lift out the barrel and the takedown-lever insert (#25). Reverse the procedure to install anew barrel. latter lift off the breech block.

When you drift the extractor pivot (#74) downward with a 1/16-inch punch, you’ll free up both extractor guides (#73), the extractor (#10), and its spring (#38). Take note of the two smaller drawings on the main schematic on the next page. The front sight on the pistol in your shop depends on the model. A Standard Mosquito has barrel (#1a) and front sight (#5a). The Sport barrel (#1b) gets a different front sight (#5b) and the Silencer Model barrel (#1c) goes with a third sight (#5c).

All front sights can be popped from the slide with the flat of a small screwdriver, and the sights vary in height according to their alphabetical suffix. For example, #5c is higher than #5b or #5a. Any barrel can be switched to another barrel, but when going from Standard to Sport, you’ll need the counterweight (#77), threaded pin (#79), and Allen key (#80) shown in the lower insert of the main schematic.

When going from Standard or Sport to Silencer (or the other way around), the assembly wrench (#82) and threaded cap (#81) shown in the lower drawing on the schematic insert come into play. Any components or tooling required to perform these barrel conversions are available from SIG Arms. Exactly how barrels are changed is included in the Mosquito owner’s manual. Inevitably, your Mosquito owner will have lost his manual, so you can refer to the drawing below for

1a Barrel 1b Barrel 1c Barrel 2 Recoil Spring Guide 3a Recoil Spring (standard velocity) 3b Recoil Spring (high velocity) 4 Slide 5a Front Sight 5b Front Sight 5c Front Sight 6 Rear Sight 7 Slide Breech Block Pin 8 Trigger Bar Spring 9 Breech Block 10 Extractor 11 Firing Pin 12 Firing Pin Spring 13 Safety Lock 14 Safety Lock Spring 15 Frame 16 Takedown Lever 17 Striker Pin for Safety Lock 18 Slide Catch Lever 19 Slide Catch Lever Spring 20 Trigger 21 Trigger Pivot 22 Trigger Bar 23 Lock 24 Sear 25 Takedown Lever Insert 26 Sear 27 Spring for Integral Lock 28 Safety Lever 29 Hammer 30 Spring for Safety Lever 31 Ball for Safety Lever 32 Ejector 33 Hammer Strut 34 Mainspring 35 Action Casing, Left 36 Ball, Centering 37 Action Casing, Right 38 Extractor Spring 39 Magazine Safety 40 Decocking Lever 41 Decocking Lever Bearing 42 Decocking Lever Spring 43 Magazine Catch 44 Manual Safety Lever, Right 45 Magazine Catch Spring 46 Manual Safety Lever, Left 47 Feeder Axle 48 Grip Plate, Left 49 Grip Plate, Right 50 Grip Plate Screws 51 Magazine Tube 52 Magazine Spring 53 Feeder 54 Magazine Floor Plate 55 Floorplate Insert 56 Threaded Socket for Grips 57 Retainer 58 Screw for Grip 59 Hammer Reset Spring 60 Hammer Pivot Pin 61 Hammer Strut Pin 62 Trigger Bar Pin 63 Spring for Magazine Safety 64 Safety Lever Pin 65 Magazine Safety Pivot Pin 66 Screw for Decocking Lever Sig Sauer Mosquito Semi-Automatic .22 Rimfire Pistol 67 Screw for Safety Lever 68 Adjusting Screw 69 Key 70 Mainspring Washer 71 Mainspring Strut 72 Key Ring (Specifications subject to change without notice) 79 Threaded Pin (b) 80 Allen Wench (b) 81 Threaded Cap (c) 82 Assembly Wrench (c) a = Standard Model b = Sport Model c = Silencer Model 73 Extractor Guide 74 Extractor Pivot Pin 75 Breech Block Pin 76 Breech Block Spring 77 Counterweight (b) 78 Cap (b)

the "how to." It’s safe to assume you’ve never come up against a handgun with a firing system like the Mosquito. Part #35 is called an "action casing" and occupies the left inner side of the frame. Its opposite casing is located inside the right side of the frame. Held between them are enough parts to make you dizzy, and they’re all vital to the safe, reliable functioning of the pistol. The frame is polymer.

The casings provide a steel bearing surface for the steel slide that’s far more durable than polymer-to-metal contact. It’s possible to remove both casings from the frame simultaneously. Remove the grips (#48 and 49). Push out the magazine catch (#43)"and be wary; within the catch are its spring (#45) and retainer (#57). The spring likes to take off on its own. Drift the trigger pivot (#21) in either direction.

Study how the decocking-lever spring (#42) and lever (#40) are joined together before removing them from the The socket screw (A) is used to secure components or a casing beneath the hold-down piece (B) during operations. left casing. Get your thumbs under both casings. Push them both upward at the same time until the casings and all they contain can be lifted free. In this united state, accumulated residue can be flushed away or cleared out with a sonic parts cleaner.

Dry the casings inside and out with compressed air or a hair dryer set on "warm." Lightly spray lubricant on both outer surfaces and inner components before reinstalling the casings. I prefer a silicone-base lube for this. Remember, the frame itself is polymer. Oil-based lubricants have been known to weaken or otherwise damage some polymers used in modernday firearms.

Worse, the misguided application of harsh solvents such as carburetor or brake-drum cleaner have actually reduced the housings of many firing systems to an unrecognizable blob. Though some polymers are immune to these harsh solvents, you can’t tell by looking. It’s always wise to contact the manufacturer or Aimporter of any gun with plastic in its construction before using a cleaning solvent. It’s wiser to simply restrict your supply of solvents to those made exclusively for sluicing out guns.

There are photos below of a special German-made jig that the factory assemblers use when building or servicing a Mosquito. I tried to secure some form of dimensional drawings. They were not available, so the photos were taken at my request to serve as a substitute. In order to work further with the casings, you’ll need a fixture like this made from a nonmarring material. High density polymer is my recommendation, except for the adjustable hold-down piece and its socket-head screw.

This particular kind of polymer machines beautifully, so milling, drilling, and tapping it present few problems. Assuming you’ll be able to build yourself a working substitute for the jig, I’m going to take you through the

The photo clearly shows how the decocking lever and its spring are engaged initially, and how they must be reengaged during reassembly. disassembly process for the casings followed by some reassembly tips. All I can say is: Without the tool or a reasonable facsimile thereof, don’t try it. On the left casing, the ejector is riveted to the casing. Enough said. Remove the slide-catch lever (#18).

Unless you’re aware of the probability, and you are as of now, the lever spring (#19) will depart to that alien world where lost parts live. As far as the magazine safety (#39) and its spring (#63) are concerned, the identical probability exists after the magazine safety-pivot pin (#65) has been drifted right to left. I figure I’d better tell you that the loss of the slide-catch lever spring can be considered assured unless the trigger-bar spring (#8) is removed first.

But you can’t remove the trigger-bar spring unless you can get at it, and you can’t get at it without the disassembly/reassembly jig. Ever heard of a Catch 22? The right casing holds most of the internal firing mechanism. Remove the mainspring-strut assembly (#34, 70, 71, and 36). Pivot the sear (#26) upward. The hammer-pivot pin (#60) trigger-bar pin (#62), trigger pivot (#21), trigger (#20), and trigger bar (#22).

Throughout the process of disassembling the casings, you must take care to examine how each inner component is attached to its related components before separating them. That will definitely make correct reassembly easier. With guns that are strange to me, like this one was, I’ve gone as far as to make careful pencil drawings or take photos to serve as guides. The grip frame is simple. Both threaded sockets (#56) for the grip panels (#48, 49) are integral with the frame itself.

Reassembly Tips The trickiest part of the reassembly begins with the right casing. Reinstall the safety-lever pin and spear the lecan be a bugger to drift, but try to do it without causing peripheral damage if at all possible. Take out the hammer (#29) and its reset spring (#59). Drift out the safety-lever pin (#64) to free the lever (#28). This is followed from the casing by the Binder Ad (p/u from page 18, AGM-August 08) ver on it.

On all newer Mosquitoes, there are very thin washers on each side of the hammer and trigger. None of these washers are shown on the schematic, but all have to be present when hammer and trigger pivot pins are reinstalled. If the hammer-strut pin (#61) has been removed, reinstall it. Slip the long end of the hammer-reset spring over the strut pin, and install the hammer on its pivot. After you slide the hammer close to the left casing, align the hammer-reset spring with its own pivot pin.

Don’t look for this pivot pin on the drawing; it’s a permanent part of the frame. You’ll find it helps to use a small punch when getting the spring into position and maneuvering its short end to tension it. Install the mainspring assembly and force the hammer into its decocked (forward) position before reinstalling the sear.

Conclusion Pardon me if I’m repeating myself here, but before you take one layer off the Mosquito’s "casing sandwich," be absolutely sure you know how the part you take off relates to the next layer. If you can come up with something to use in place of the factory fixture, and if you’re very careful with the disassembly, you might be able to suppress the urge to dispatch this Mosquito with a large dose of flyinginsect killer. I