The Sig P320 Full-Size .357 SIG is the same modular striker pistol the U.S. Army adopted as the M17 in 9mm, with a different barrel chambering. Same removable fire control unit, same full-size grip module, same 4.7" barrel length, same 8.0" overall length. Capacity is 14+1 in .357 SIG versus 17+1 in the Sig P320 Full Size 9mm, since the bottleneck cartridge takes more room in the magazine. At 29.4 oz unloaded, the gun runs the same external dimensions as the 9mm version with the .357 SIG barrel and a recoil spring tuned for the higher chamber pressure.

The P320's serialized fire control unit is the platform's defining feature. The trigger group lifts out of the polymer grip module and drops into a different frame size, so one registered component covers full-size, carry, compact, and subcompact configurations. Sig also sells X-Change Kits that convert the same fire control unit between 9mm, .357 SIG, and .40 S&W with barrel, slide, magazines, and recoil spring all in one package. This is unique in the striker-fired market. The Glock G31 Gen4 .357 SIG requires a separate registered pistol per caliber; the P320 needs only one.

The 6.5 lb trigger is the P320's main weakness in the .357 SIG configuration, the same trigger as the 9mm version, just heavier than the 5.5 lb Glock G31 trigger and meaningfully heavier than the Sig P226 .357 SIG's 4.4 lb single-action pull. SIGLITE night sights ship standard, the Picatinny rail accepts standard duty lights, and the optic-ready slide takes common red dot footprints with the included plate system. The stainless steel slide and 29.4 oz weight produce noticeably softer felt recoil than the polymer-framed Glock G31, which weighs 26.1 oz. That 3.3 oz difference shows up over a string of shots, especially with 125gr loads near the cartridge's upper velocity range. The P320 fits the shooter who wants a duty-length .357 SIG with a striker trigger and the option to convert calibers later without buying another pistol.