The Glock 31 Gen4 is a Glock 22 with a .357 SIG barrel. Same 4.49" Marksman-pattern barrel length, same Picatinny rail, same 15-round magazines that work in either caliber. .357 SIG was developed in 1994 by Sig Sauer and Federal as a bottlenecked round derived from the 10mm Auto case (necked down to 9mm bullet diameter), aimed at duplicating .357 Magnum ballistics in a semi-auto. It hit hard for a few years with state troopers and the Secret Service, then lost ground to 9mm as defensive ammunition improved. The G31 is what's left of that era for Glock — still in production, still chambered in the cartridge that some agencies and individual shooters keep running for its flat trajectory and barrier performance.

At 26.1 oz unloaded, the G31 weighs about 2.3 oz less than the Glock G22 Gen5 .40 S&W at 28.43 oz, even though the frame, slide dimensions, and magazine well are interchangeable. The .357 SIG barrel is the only meaningful difference. Within the .357 SIG category, the Sig P226 .357 SIG runs 34 oz with an alloy frame and DA/SA trigger — substantially heavier and a different manual of arms entirely. The Sig P320 Full-Size .357 SIG sits between the two at 29.4 oz with a striker trigger.

The harder buying question is not which .357 SIG to pick. It's whether to pick .357 SIG at all. The G31 shares its frame with the Glock G17 Gen5 9mm, which runs lighter at 24.87 oz with 17+1 capacity, costs less to feed by a wide margin, and has a deeper ammunition selection. Buy the G31 if you have a specific reason to be in .357 SIG: barrier performance, departmental compatibility, or you simply like the cartridge's flatter trajectory at 25 yards. Buy a G17 if you don't.