The P250 is the second polymer-framed pistol from SIG-Sauer, after the SIG Pro. The hammer-fired P250 is also double-action only and allows for caliber changes.
Like other SIG-Sauer pistols, the P250 pistol is a short recoil operated, locked breech pistol. It uses the typical SIG-Sauer cam-operated barrel, which locks into the ejection port on the slide. A key feature of the pistol is its modular design. The main module of the P250 is an internal stainless steel frame fire control unit which consist of the integrated fire control group (trigger, hammer and all necessary linkage and springs), ambidextrous slide release (one on each side of the pistol), ejector and four slide rails. This steel receiver unit is used in every P250 grip module, top end, and chambering combination and is inserted into the grip frame module and held in place by the axis pin of the disassembly lever. The firearm's serial number is on the fire control unit instead of the grip module. The fire control unit allows the operator to interchange differently-sized slide assembly or top end modules and polymer grip frame modules and trigger shoes and (limited) chambering conversions to adapt the gun to the individual user and address different needs. The P250 was designed to be ambidextrous in handling, sporting an ambidextrous slide catch lever and user reversible magazine catch. All other operating controls are designed so they can be operated from either side. The firearm can be field stripped with no tools. The full-size and compact grip modules feature an integral Picatinny rail mounting-bracket at the lower forward edge of the module to allow the mounting of laser sights, tactical lights, and other accessories. Before the introduction of the Picatinny rail SIG used a differing Rail Integration System on the P250 Compact.
The P250 pistol is offered in three glass-reinforced polymer grip module sizes—small, medium, and large—in each of the full-sized, compact, and subcompact P250 variants. The pistol top ends also come in full-sized, compact, and subcompact variants that will fit the correspondingly sized grip modules. Every chambering can be converted by the user with a caliber exchange kit. The 9×19mm Parabellum, .40 S&W, and .357 SIG chamberings share their polymer grip frame modules and this offers the possibility to interchange these three chamberings by swapping the top end—consisting of the slide, recoil spring assembly, barrel and the box magazine. Due to the significantly larger external dimensions of the .45 ACP cartridge compared to the other P250 chambering options, the .45 ACP chambering uses slightly wider .45 ACP specific polymer grip modules with an enlarged magazine well. The manufacturer offers caliber exchange kits or Caliber X-Change kits consisting of a slide, recoil assembly, barrel, box magazine and grip module in the varying frame sizes, grip module sizes and chambering variants in which the P250 pistol is offered. These kits enable a user to switch between the various P250 configurations without the use of tools.
The P250 has no manual safeties to promote instant readiness. Instead of manual safeties an automated firing pin safety ensures safe carrying of the weapon whilst preventing accidental discharges if the gun is dropped or roughly handled.
The trigger is a self-decocking DAO trigger system with spurless and recessed hammer. The trigger system has a pull weight of about 5.5 lbf (3.9 kgf, 38 N) and will not reset until completely released. The P250 trigger is available in standard or short configurations meaning two user interchangeable trigger shoe variants are offered. The short reduced reach trigger shoe configuration can be reached and operated better by users with relatively short fingers.