One of the great things about the Glock platform is that, with the possible exception of the Colt 1911-series pistol, they have just about the largest selection of aftermarket add-ons in the world. Besides performance and cosmetic parts, there are also sub caliber kits that can drop the chambering down to something a little...less.
What are they?
Simply put, these conversion kits swap take your standard Glock pistol, and after swapping out the loaded slide (which includes the barrel, and recoil spring/rod) and magazine, will then shoot .22LR ammunition. As such, as long as you can field strip your gun for cleaning, you can install one of these kits on your frame. Then, when you are ready to return the gun to its original caliber, just perform the same procedure in reverse, as it creates no permanent change to the Glock.
Why would you even want to do this?
Converting your larger caliber (9mm/.40S&W, etc.) pistol to the smaller .22LR round, can ease introduction by young and first time shooters to a full sized handgun due to the fact that the lower powered rimfire round has far less recoil and muzzle flip. Use of these smaller rounds is also beneficial to ranges as they cause less wear and tear on target stands, berms, etc.
Besides, there (used to be) a significant savings in firing rimfire rounds over standard rounds, and even though today's prices are about 300-400% higher than what was out there just a few years ago, .22LR is starting to come back on the market to one degree or another and prices are starting to fall.
Downsides of these kits
They are one of the pricier of aftermarket accessories for the Glock, with the two most common kits, by Advantage Arms and Tactical Solutions, running between $265-$395 depending on the vendor and model type. Ciener kits are also out there for a little cheaper ($200-ish) but you get the idea. For that price, you can get a dedicated .22LR plinker pistol.
In addition, from most of what we have seen these kits (as are most .22LR semi-autos) are very ammo-sensitive. They seem to like your spicier ammo (Federal Game-Shok, CCI Mini Mags, Stinger, Velocitor, etc.) while 'bulk ammo' like the infamous Remington Golden bullet and pipsqueak rounds such as the Colibri low velocity load are almost guaranteed to be hand-fed one shot rounds. However, this is good to keep in mind as using them, especially if mixed into random mags, can be a nice segway into fast and effective Tap-Rack-Assess malfunction drills.
With those caveats, let us look at what is out there.
These kits are some of the most popular as they are advertised as being 'last round hold open' and have standard Glock sights, which makes these the choice of shooters looking to use a kit to increase trigger time while retaining the same sight picture. Further, AA sells these in a half dozen different models for both classic and 4th Gen Glocks with standard, compact and subcompact frames. Got a Gen 4 G20? AA has a kit for it to keep from burning through that precious 10mm.
(Glock Store video of the Advantage Arms kit in action)
Tactical Solutions Kits
Another major player in the Glock subcaliber kits is Tacsol. Known for a series of Ruger and Browning upgrades as well as .300BLK and suppressor technology, the company sells a series of .22 conversions for the Glock. These steel-slide kits come in fewer options than those by Advantage Arms, and as a whole are slightly higher priced-- but they offer a series with threaded barrels, which is nice for those who would like some quiet time
A quick Gunblast review of the TS kit
Of course, Glock could always just get on the bandwagon and start making a .22LR safe action pistol.
And when they eventually do, we'll be all over it, but until then...