The SIG Sauer MPX 4.5" is an ultra-compact 9mm platform, frequently referred to as the MPX-K, designed as a highly modular and reliable sub gun for personal defense, home defense, and close-quarters use. Unlike many 9mm carbines that use direct blowback systems, the MPX utilizes a closed-bolt, short-stroke gas piston system, which significantly reduces recoil and enhances reliability

Barrel & Operation: 4.5-inch carbon steel barrel, often with a 13.5x1mm LH thread pitch (in older models) or more standard threading on newer ones, utilizing a short-stroke piston.

Trigger: Equipped with a Timney single-stage trigger, co-engineered with SIG for a crisp break and fast reset.

Controls: Fully ambidextrous, including the bolt release, safety selector, and charging handle.

Handguard: M-LOK aluminum handguard (Generation 2) for accessory mounting.

Capacity: Ships with 30-round or 35-round steel-reinforced polymer magazines.

Stock/Brace: Often features a folding PCB (Pistol Contour Brace) or, in the true SBR configuration, a stock.