P320 Compatibility Guide


Sig Sauer P320 Complete Buyer & Variations Guide

StrikePoint Gunsmithing – East Tennessee

The Sig Sauer P320 is one of the most modular, customizable, and widely adopted striker-fired pistols in the world. With its serialized Fire Control Unit (FCU), interchangeable grip modules, slide lengths, calibers, and countless variants, the P320 has become the AR-15 of modern handguns—an endlessly configurable platform.

But that modularity also creates confusion for buyers:

  • Which P320 models are full-size, compact, or carry?
  • What slides and barrels are interchangeable?
  • How do the X-Series and Legion models differ?
  • What’s the deal with the M17/M18 military pistols?
  • Which grip modules and triggers fit which FCUs?

This guide breaks it all down with clarity and technical accuracy from a gunsmith’s perspective.

What Makes the P320 Unique?

The Serialized Fire Control Unit (FCU)

The P320’s entire identity is built around the Fire Control Unit (FCU), which is the serialized “firearm.” The FCU houses:

  • The trigger and trigger bar
  • Sear and striker safety parts
  • The internal rails the slide rides on

Everything else—grip module, slide, barrel, magazines—can be swapped, allowing you to convert one FCU into multiple pistol configurations.

You can build:

  • A compact carry pistol
  • A full-size duty gun
  • A competition setup
  • A home-defense pistol with optic and light
  • A custom build with premium Icarus, Wilson, or Mischief Machine modules

The FCU is universal in concept, but not all FCUs are identical—especially when it comes to safety updates and internal geometry revisions.

The Four Main P320 Sizes (Grip Module Families)

Understanding the P320 starts with its grip modules. There are four major size families:

Full-Size (FS)

  • Longest grip
  • Uses 17-round magazines
  • Pairs with the full-size 4.7″ slide
  • Ideal for duty, home defense, and competition

Carry

  • Same full-height grip as Full-Size
  • Uses 17-round magazines
  • Shorter dust cover than Full-Size
  • Typically paired with a 3.9″ Compact slide

Compact

  • Shorter grip than the Carry model
  • Uses 15-round magazines
  • Pairs with the 3.9″ Compact slide
  • Best for concealed carry users wanting full ergonomics

Subcompact (Legacy)

  • Shortest grip module
  • Uses 12-round SC magazines
  • Works only with older Subcompact slides
  • Superseded by the P365 line for most users

Fitment Tip: Compact and Carry share the same slide length (3.9″) but differ in grip height. Full-Size slides do not fit Compact or Carry frames due to dust-cover mismatch.

Slide Lengths & Barrel Options

The P320 platform has four main slide lengths:

Full-Size Slide – 4.7″

  • Requires Full-Size grip modules
  • Not compatible with Compact or Carry grips

Compact / Carry Slide – 3.9″

  • Works on Compact and Carry grip modules
  • Does not fit Full-Size grip modules

X-Compact Variation

Similar to Compact length but may use a different recoil assembly. Designed for X-Series grip modules.

Subcompact Slide

A legacy option. Works only with Subcompact grip modules. Not compatible with Compact, Carry, or Full-Size grip modules.

The X-Series Explained

The X-Series of P320 pistols introduced major upgrades to ergonomics, trigger feel, and performance.

X-Series Benefits

  • Flat X-Series trigger
  • Improved undercut and beavertail
  • Enhanced grip angle and ergonomics
  • Better texture on grip modules
  • Optics-ready slides on most models

X-Full

Full-size slide + full-size grip. Uses 17-round mags. Ideal duty setup.

X-Carry

Compact slide + full-size height grip. Uses 17-round mags. One of the most versatile P320 variants.

X-Compact

Compact slide + shorter Compact grip. Uses 15-round mags. Excellent for concealed carry.

X-Five (Competition)

Includes bull barrel, skeletonized trigger, optic-ready slide, and weighted grip for competition.

X-Five Legion

Features the tungsten-infused TXG grip module for reduced recoil. One of the top competition striker-fired pistols.

M17 & M18 Military Pistols

The U.S. military adopted two P320 variants, both finished in coyote-colored parts:

M17

  • Full-size slide
  • Carry-length grip
  • Often includes a manual safety
  • Optics-ready with DeltaPoint Pro footprint

M18

  • Compact (3.9”) slide
  • Carry-length grip
  • Same military-inspired coyote finish

Civilian versions are widely available with and without manual safeties.

FCU Generations, Safety Updates & “Upgraded” vs “Original” P320

The P320 has undergone major internal safety improvements:

Pre-Upgrade FCUs (Older Versions)

  • Heavier trigger shoe
  • Different striker geometry
  • No disconnector
  • Known drop-safety concerns

Post-Upgrade FCUs (Modern Versions)

  • Lighter, smoother trigger
  • Two-stage striker safety
  • Added disconnector mechanism
  • Improved sear housing geometry

If buying used or starting a build, verify your FCU is post-upgrade. StrikePoint can inspect and confirm this for owners.

Optics Mounting: Footprints & Fitment

The P320 uses multiple optic footprint standards depending on the model:

  • DPP / R1P footprint (DeltaPoint Pro)
  • RMSC footprint (common on X-Compact and slim models)
  • ProCut footprint (used on some M17/M18 models)
  • Romeo1 cut (older P320 slides)

Future StrikePoint articles can provide detailed optic compatibility charts across the P320 lineup.

Magazine Compatibility

The good news: P320 magazine compatibility is simple.

Full-Size & Carry Mags – 17 Rounds

  • Work in:
    • Full-Size
    • Carry
    • X-Carry
    • X-Full
    • Compact (with grip sleeves)

Compact Mags – 15 Rounds

  • Fit Compact and X-Compact
  • Can be used in Carry/Full-Size but won’t be flush

Extended 21-Round Mags

  • Fit all P320 9mm grip modules
  • Great for range, home defense, and competition

Gunsmith’s Notes & Common Issues

  • Trigger bar & sear housing alignment: Aftermarket grip modules may introduce tolerance issues.
  • Extractor tension: A common source of erratic ejection.
  • Recoil spring mismatches: Mixing slide types requires careful recoil spring selection.
  • Manual vs non-manual safety FCUs: Manual safety FCUs require matching grip modules and slides.
  • Cheap aftermarket parts: Often lead to reliability issues. Stick with OEM, Grayguns, Wilson, Icarus, Mischief Machine, and other reputable brands.

How This Fits Into the Sig Sauer Resource Hub

This P320 guide serves as a pillar page for your Sig Sauer Resource Center. From here, you can build out a full cluster of related articles, such as:

  • P320 Grip Modules Explained (OEM + Aftermarket)
  • P320 Trigger Options Ranked by a Gunsmith
  • Best Lights for P320 Duty & Carry
  • Which Optics Fit Which P320 Slides
  • P320 Safety Update: How to Tell If Yours Is Upgraded
  • P320 M17 vs M18: Complete Comparison

Internal linking these articles will help make StrikePoint the leading online authority for Sig Sauer gunsmithing knowledge.

If your P320 needs professional inspection, tuning, or configuration help and you’re in East Tennessee, we’re here for you.

Contact StrikePoint Gunsmithing to schedule service or ask a question.

Need Help With Your Sig?

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