The Glock 31 is a pistol produced and manufactured by the Austrian Glock Company. It is essentially a Glock 22 modified to fire the larger .357 SIG ammunition.
The G31 uses a modified frame, slide, magazine and .357 SIG barrel but is otherwise nearly similar in design to the Glock 22. It has a large 15-round magazine capacity. (A Glock 31 can simply be converted to a .40 S&W with the simple modification of a new barrel.)
The Glock 31 has become an extremely popular device within the sports and hunting community as well as the law enforcement community, but is consistently considered to be less comfortable to shoot than the Glock 17 because of its quick and sharp recoil.
The unique difference between the Glock 31C and the Glock 31 is that the slide and barrel are ported on the Glock C model to drastically reduce muzzle climb while firing the pistol.