A 10-round version of this magazine is also made for markets that restrict the magazine capacity of handguns. And, a "+2" base plate can make the standard magazine into a longer 17-round magazine. The pistol is also compatible with any magazines designed for the Glock 17 and Glock 18, providing factory magazine capacities of 17, 19, 24 and 33. Changing out base plates adds capacities of 26 or 31. To preserve the operational reliability of the short recoil system, the mass of the slide remains the same as in the Glock 17 from which it is derived. With the exception of the slide, frame, barrel, locking block, recoil spring, guide rod, and slide lock spring, all of the other components are interchangeable between the models 17 and 19. The Glock 19 Gen 4 MOS (Modular Optic System) has also been used by Special Operations Forces as the MK27 MOD 2. One of the oldest of the Glock pistols, options like threaded barrel and MOS slide cuts are available, and suffixed model designations have been created for some features, all of which excepting the G19X can use any magazine the G19 can.