These barrels allow you to convert your existing Glock to shoot another caliber. For example:
Your Glock 22 can be converted to shoot 9mm ammunition by simply replacing your existing barrel with this conversion barrel and using a Glock 17, 9mm magazine.
Or, your Glock 20 can be converted to shoot .40 S&W.
This is an excellent way to get the most from your Glock handgun and save money on ammo. Make sure to get some new matching magazines to go along with your new caliber barrel.