SIG Sauer Parts Compatibility: What Fits the P320, P365 and P226

Most SIG Sauer parts do not interchange between pistol lines, and the ones that look like they should are usually the ones that do not. The P320 and P365 share a design philosophy but no major components. The P226 shares almost nothing with either. The M17 and M18 are P320 variants and share a great deal.

This guide maps what actually fits what, so you can order the right part the first time.

Key Takeaways

• Architecture determines compatibility. The P320 and P365 are striker-fired, fire-control-unit pistols. The P226 is a hammer-fired, all-metal-frame pistol. They are unrelated platforms.
• P320 and P365 parts do not interchange. Same concept, different scale. Nothing meaningful crosses over.
• M17 and M18 are P320 variants. Most P320 parts apply, with exceptions around the manual safety and optic provision.
• Within a line, size class still matters. A P320 grip module must match the fire control unit size and slide it will run.
• Always verify by part number. Generational running changes mean a part that fits one production run may not fit another.

The Three Architectures

Understanding why things do not fit is faster than memorising what does.

The P320 and P365 both use a removable serialised chassis inside a replaceable grip module. That shared concept is what misleads people. The chassis dimensions, slide rails, trigger geometry, magazine well and lockwork are all different between the two. They are cousins, not siblings.

The P226 predates both by decades and works on an entirely different principle — a hammer, a decocker, a double-action first shot, and a frame that is itself the serialised firearm. Nothing from the striker-fired lines applies.

P320: What Fits

The P320 is the most modular pistol we stock, and the deepest part of our catalogue.

Grip Modules

Grip modules are specified by size class and must match the fire control unit and the slide configuration. The common classes are Full Size, Carry, Compact, Subcompact, and the X-series variants including X-Carry and X-Five.

Sizes are not freely mixable. The module houses the chassis and presents the magazine well; a mismatch between module, chassis length and magazine is not a preference question, it is a function question. SIG documents the intended combinations per module, and those are the combinations to order against.

Material variants — standard polymer, AXG aluminium, TXG tungsten-infused — are interchangeable within a size class. An AXG X-Carry module replaces a polymer X-Carry module directly. See our P320 grip module guide for how the materials actually differ in the hand.

Magazines

P320 magazines are specified by calibre and size class. Full Size and Carry modules commonly run 17-round 9×19 magazines; Compact runs 15; Subcompact runs 12; X-Five runs 21.

A longer magazine will generally function in a shorter grip module with the baseplate protruding. The reverse does not work — a short magazine in a long grip leaves the feed lips below the intended position.

One legal note that belongs here rather than in a footnote: under Directive 2017/853, a detachable loading device exceeding 20 rounds inserted into a centre-fire semi-automatic short firearm moves it into Category A. The 21-round X-Five magazine crosses that line. Several member states also apply stricter national capacity limits. Check your authorisation before ordering — our EU cross-border transfer guide covers how component transfers differ from firearm transfers.

Slides and Optics

P320 slides are specified by size class and by optics cut. SIG's own footprints — ROMEO1PRO and the newer ROMEO-X pattern — dominate the factory-cut slides, with adapter plates available for common third-party footprints.

Buy the slide for the optic you own. Plate stacking raises the sight above the bore and complicates co-witness with suppressor-height irons.

Small Parts

Springs, extractors, slide catches, takedown levers, grip screws and magazine release assemblies are specified by model and often by calibre. A P320 extractor for 9mm/.357 SIG/.40 is not the .45 Auto part. These are the items most often ordered incorrectly, and the part number is the only reliable identifier.

Browse the full SIG P320 parts range.

P365: What Fits

The P365 family shares the fire-control-unit concept with the P320 and shares no major parts with it.

Within the family, the variables are grip length, slide length and optic provision:

Holsters and magazines are the two categories where people cross these up. A holster moulded for a standard P365 will not correctly cover an XL's longer slide. Magazines follow the same directional rule as the P320 — longer into shorter works with a protruding baseplate, shorter into longer does not.

See P365 parts and holsters and our P365 holster and setup guide.

M17 and M18: P320 Variants

The M17 and M18 are the military-pattern P320s — full size and compact respectively — and most P320 compatibility carries over. The exceptions cluster around two features:

• Manual safety. M17/M18 grip modules and chassis accommodate a manual thumb safety. A standard P320 module without safety provision is not a drop-in replacement, and vice versa.
• Optic provision. These ship optics-ready, and the cut determines which sight mounts directly.

Treat them as P320s with a specification qualifier, not as a separate line.

P226 and P229: The Separate Family

The P226 is hammer-fired with a frame-serialised design. Nothing from the P320 or P365 applies — not magazines, not grips, not slides, not springs.

Within the family, the P226 and P229 share design lineage but differ in frame size and, importantly, in magazines. P226 and P229 magazines are not interchangeable across all calibres and generations. Grips — the panels on the sides of the frame — are model-specific.

Browse the P226 range.

MCX: A Different Problem Entirely

The MCX is a rifle platform and follows rifle conventions. It uses an AR-pattern fire control group and AR-pattern pistol grips, and 5.56 variants feed from standard STANAG-pattern magazines — but the upper receiver, bolt carrier, handguard and stock interface are proprietary.

The practical rule: AR-15 trigger and grip parts often apply; anything structural does not.

Note the Category A implication for long firearms. Under Directive 2017/853, a centre-fire semi-automatic long firearm falls into Category A where it allows firing more than 11 rounds without reloading with a loading device exceeding 10 rounds, and separately where a folding or telescoping stock reduces it below 60 cm overall without loss of function. Both are relevant to MCX configurations. Check your authorisation.

How to Order the Right Part

• Identify your exact model and generation, not just the line. "P320" is not enough; "P320 Compact 9mm, optics-cut" is.
• Use the part number where you have it. Model names are marketing; part numbers are engineering.
• Check calibre on every internal part. Extractors, ejectors and springs commonly differ between 9mm/.357/.40 and .45 Auto.
• Match the optic footprint before buying a slide, not after.
• Confirm magazine capacity against your permit category before ordering, especially for X-Five and MCX configurations.

If you are unsure, send us your model and serial range and we will confirm fitment before you order.

Reading a SIG Part Number

Model names describe marketing segments; part numbers describe parts. When a listing and a model name disagree, the part number wins.

What to capture before ordering:

• The model and variant in full — "P320 Compact", not "P320"
• The calibre — many internals differ between 9mm/.357 SIG/.40 and .45 Auto
• The generation or production range — running changes are common and undocumented in marketing material
• The optic footprint, if the part touches the slide
• Whether the pistol has a manual safety, which changes chassis and module parts

If you have the pistol in front of you, the slide legend and the fire control unit marking give you most of this. If you are ordering for a pistol you have not yet received, ask the selling dealer for the exact configuration rather than inferring it.

Optic Footprints in Practice

Footprint mismatch is the most expensive compatibility error because it is usually discovered after the slide has been bought.

SIG's own patterns dominate its optics-ready pistols, principally the ROMEO1PRO footprint and the newer ROMEO-X pattern on current-generation guns. Adapter plates exist for common third-party footprints, and they work, but each plate adds height between bore and optic.

The rules that avoid trouble:

• Choose the optic before the slide.
• Prefer a direct-mount footprint match over a plate where one exists.
• If you must use a plate, check that suppressor-height irons still co-witness through the window.
• Confirm your holster is cut for the resulting profile.

Wear Items and Replacement Intervals

Compatibility questions cluster around upgrades, but most parts orders are maintenance. The items that actually wear:

None of these are expensive. All of them are calibre- and model-specific, which is why they belong in a compatibility guide at all. Order by part number, and keep spares of the first two.

Browse springs, extractors and parts kits.

The Five Most Common Ordering Mistakes

• Assuming P320 and P365 interchange. They do not. Different platforms entirely.
• Ordering by model name without calibre. Internals differ, particularly .45 Auto against the 9mm/.357/.40 family.
• Buying a slide before choosing an optic. Footprint drives the slide, not the reverse.
• Mixing grip module and chassis size classes. Module size must match the fire control unit it houses.
• Ordering magazines without checking permit category. Capacity has legal consequences under Directive 2017/853 and under stricter national rules. See our EU cross-border transfer guide.

Frequently Asked Questions

Will P320 magazines work in a P365?

No. Different platforms, different magazine dimensions and different lockwork. Nothing crosses over.

Are M17 parts the same as P320 parts?

Mostly, with exceptions around the manual safety and the optics cut. A grip module or chassis with safety provision is not interchangeable with one without.

Can I put an X-Five grip module on a Compact fire control unit?

Grip modules must match the chassis size class and the slide they will run. Combinations outside those SIG documents are not a matter of preference. Check fitment before ordering.

Do P226 magazines fit a P229?

Not reliably across calibres and generations. These are related but distinct frames. Order by model and calibre.

Why do you stock so many P320 and P365 parts specifically?

Because they are the platforms our customers actually shoot. The P320 and P365 together account for the large majority of our accessory catalogue, which is why compatibility questions concentrate here.

Can parts be shipped across EU borders more easily than firearms?

Generally yes — most parts are not controlled components. Magazines are the significant exception and are restricted independently in several member states. See the EU cross-border transfer guide.

The Short Version

Architecture first, size class second, part number third. The P320 and P365 share an idea and nothing else. The M17 and M18 are P320s with a specification. The P226 is a different pistol from a different era. The MCX is a rifle that borrows AR fire control and nothing structural.

Browse the full SIG Sauer range, the parts and accessories catalogue, or ask us to confirm fitment before you order.

Nothing here is legal advice. Magazine capacity classifications under EU Directive 2017/853 and stricter national rules are your responsibility to verify with your national authority.