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Last updated March 20th 2026
The SIG P320 is the most customizable handgun on the market, and it’s not particularly close. The entire gun is built around a modular fire control unit (FCU) that pops in and out of different grip modules, slides, and barrel configurations. One serial number, infinite builds. Want a full-size range gun on Monday and a subcompact carry gun on Friday? Same FCU, different parts. No other pistol does this.
We compare SIG P320 parts from Brownells, Palmetto State Armory, MidwayUSA, Optics Planet, and 80+ other retailers so you can find the best price on grip modules, slides, barrels, triggers, and everything else. Filter by part type, brand, and price. Every link goes straight to the retailer.
The P320 platform was chosen by the U.S. military as the M17 and M18 service pistols, which means parts availability and aftermarket support are going to stay strong for decades. If you’re investing in this platform, you’re investing in something with a long future.
-76%Hogue HandAll Beavertail Grip Sleeve Sig Sauer P320 Compact Black
-56%Wilson Combat Sig Sauer P320 Slide Release Post and Kit
Wcp320 Magazine Release Spring & Stop Kit - Wcp320 Magazine Release Spring And Stop Kit
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-61%AMEND2 SIG P320 A2 17 ROUND BLACK MAGAZINE DESIGNED FOR USE IN SIG SAUER P320 FULL-SIZE 9MM FIREARMS
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P320 Subcompact Inner Recoil Spring - Sig Sauer 320 Compact Inner Recoil Spring
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ETS Sig Sauer P320 / P250 Compact 9mm 15-Round Carbon Smoke Magazine
P320 Subcompact Inner Recoil Spring - Sig Sauer 320 Compact Inner Recoil Spring
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Elite Tactical Systems Sig P320 9mm 17 Round Carbon Smoke Magazine
Obsidian Arms Sig Sauer P320 Retainer Plate Round Peg Black
Obsidian Arms Sig Sauer P320 Retainer Plate Square Peg Black
ETS Sig Sauer P320 / P250 9mm 17-Round Carbon Smoke Magazine
Sig Sauer 320 Magazines 9mm - Magazine Standard 17-Rd 9mm For Sig Sauer P320 Carbon Smoke
Why the P320 Is the Most Customizable Handgun You Can Buy
Every other pistol on the market treats the frame as the firearm. Want a different size? Buy a different gun. Want a different caliber? Buy a different gun. The P320 threw that out the window. The fire control unit (the chassis that holds the trigger group) is the serialized firearm. The grip module, slide, and barrel are just parts. Unregulated, no-FFL-required parts that you can swap in 30 seconds.
I’ve built three completely different P320 configurations from a single FCU. A full-size with an X-series slide and Wilson Combat grip for the range. A compact X-Carry setup for everyday carry. And a subcompact for deep concealment when I need it. One serial number in the safe, three different guns depending on the day. No other platform in the handgun world offers this kind of flexibility.
The military adoption as the M17 (full-size) and M18 (compact) means this platform isn’t going anywhere. SIG and the aftermarket are heavily invested, and parts availability keeps getting better. It’s the closest thing the pistol world has to the AR-15’s ecosystem.
SIG P320 Fire Control Unit (FCU): The Heart of the System
The FCU is where every P320 build starts. It’s the stainless steel chassis that houses the trigger, sear, safety mechanisms, and the serial number. This is the only part of the P320 that legally constitutes a “firearm” and requires an FFL transfer. Everything else ships to your door like any other online purchase.
Standalone FCUs retail for around $300 to $350, and they occasionally dip under $300 during sales. If you’re building a P320 from scratch rather than buying a complete gun and swapping parts, buying a standalone FCU and adding the components you want is the most efficient approach. You get exactly the configuration you want without paying for a complete gun’s worth of parts you’re going to replace anyway.
The FCU is compatible across the entire P320 family in 9mm, .357 SIG, and .40 S&W. Drop the same FCU into a full-size grip module with a 4.7″ slide or a subcompact grip with a 3.6″ slide. It doesn’t care. The .45 ACP P320 uses a wider FCU that is not compatible with the standard 9mm/.40/.357 lineup.
P320 Grip Modules: The Biggest Upgrade You Can Make
The grip module is where the P320 really shines as a customizable platform. Swapping grip modules changes the size, feel, weight, and ergonomics of the entire gun. And it takes about 30 seconds: pop out the takedown lever, lift the FCU out, drop it into the new grip. No tools required.
Standard polymer grip modules come in Full-size, Carry, Compact, Subcompact, X-Carry, and X-Compact configurations. The X-series grips have a higher undercut on the trigger guard, a flat-bottomed magwell, and a more aggressive texture. They’re a noticeable improvement over the original smooth grips for around $40 to $50.
The Wilson Combat grip module is the single best $65 you can spend on a P320. Full stop. The stippling pattern is aggressive without being uncomfortable. The grip angle is slightly steeper than factory. The undercut is deeper. It turns a good gun into a great one. I’ve put Wilson Combat grips on two of my P320 builds and have zero desire to go back. Available in Carry and Full-size configurations, with and without manual safety cut.
The AXG (Alloy X Grip) module is SIG’s aluminum-framed option. It’s heavier, which absorbs recoil, and has a premium metal feel that polymer can’t match. At $300 to $400 it’s a significant investment, but if you want your P320 to feel like a metal-framed pistol, this is how you get there. The TXG (Tungsten-infused X Grip) is even heavier and designed for competition shooters who want maximum weight to reduce muzzle rise. At around $250, it’s actually a better value than the AXG for pure weight.
Slides and Barrels: Changing Your P320’s Personality
The slide and barrel determine the size, weight balance, and optics capability of your P320 build. SIG offers several factory options, and the aftermarket is growing fast.
Factory slide options: The standard P320 slides are reliable but lack an optic cut. The X-series slides come pre-cut for SIG’s Romeo1Pro/Romeo2 optic footprint, which is proprietary and annoying if you want to run a non-SIG optic. The Pro-Cut slides have lightening cuts for reduced weight and faster cycling. The Legion slides are the premium factory option with refined serrations and an enhanced finish.
Aftermarket slides from Grey Ghost Precision, Zev Technologies, and others typically use the RMR footprint instead of SIG’s proprietary cut. This means you can run a Trijicon RMR, Holosun 507C/407C, or any other RMR-pattern optic without adapter plates. If you want to run a non-SIG red dot (and most people do), an aftermarket optic-ready slide is the way to go.
Barrel lengths correspond to frame sizes: 3.6″ for subcompact, 3.9″ for compact and carry configurations, and 4.7″ for full-size. You can technically put a shorter barrel in a longer slide (it’ll just have a gap at the muzzle) but not a longer barrel in a shorter slide. Threaded barrels are available in all lengths for suppressor or compensator use. SIG’s factory threaded barrels are good. True Precision and Faxon make excellent aftermarket options.
P320 Triggers and Small Parts
The stock P320 trigger is decent. The curved version is perfectly functional for a duty or carry gun. But swap in a flat-face trigger and the difference is immediately noticeable. The flat face gives you a more consistent pull because your finger contacts the trigger at the same point every time regardless of where on the trigger you place it.
SIG sells their own flat-face trigger as a factory upgrade, but the Apex Tactical flat trigger is the aftermarket favorite at around $85 to $100. Cleaner break, shorter reset, and a better overall feel than the SIG factory flat trigger. It’s the first thing I’d swap on any P320. The Armory Craft adjustable trigger is the other strong option at $70 to $90, with adjustable pre-travel and over-travel that lets you tune the trigger to your preference.
For small parts, the P320 is relatively low-maintenance. Recoil springs should be replaced every 3,000 to 5,000 rounds. Striker springs and trigger return springs may need attention around 10,000+ rounds. SIG OEM replacement parts are affordable and readily available. Keep a spare recoil spring assembly on hand if you shoot regularly.
Caliber Conversion Kits: One Gun, Multiple Calibers
This is one of the P320’s most underappreciated features. Because the FCU is caliber-agnostic (in the 9mm/.357/.40 family), you can convert between calibers with minimal parts swapping.
9mm to .357 SIG: The easiest conversion. Just swap the barrel. The slide, recoil spring, and magazines are the same between 9mm and .357 SIG. Drop in a .357 SIG barrel and you’re done. This is possible because .357 SIG is a necked-down .40 S&W case that uses the same overall cartridge length as 9mm.
9mm to .40 S&W: Requires a barrel and extractor swap at minimum. A complete .40 caliber slide assembly is the cleaner path because it includes the correct extractor and breech face. Magazines are the same as .357 SIG mags (they share the same case diameter).
.45 ACP: This is the big exception. The P320 in .45 ACP uses a wider frame and FCU that is NOT compatible with the 9mm/.40/.357 family. If you want a .45 ACP P320, it’s essentially a separate gun. Parts do not interchange between the .45 platform and the standard platform.
Caliber conversion kits (slide + barrel + recoil spring as a package) are the easiest way to convert. SIG and several aftermarket manufacturers sell them. Use the price comparison grid above to find the best deal on conversion kits across 80+ retailers.
P320 Magazines: Capacity and Compatibility
P320 magazines come in three standard capacities: 15-round (compact/carry), 17-round (full-size), and 21-round extended. Here’s the compatibility rule: longer magazines work in shorter grip modules (they’ll just stick out the bottom), but shorter magazines won’t seat in longer grip modules without a baseplate adapter.
Fun fact: Mec-Gar manufactures SIG’s OEM magazines. So when you see Mec-Gar P320 magazines at a lower price than SIG-branded ones, you’re getting essentially the same magazine without the SIG rollmark. They’re excellent and typically $5 to $10 cheaper per magazine.
For states with magazine capacity restrictions, 10-round versions are available in all configurations. The 21-round extended magazines are great for range use and competition, adding five extra rounds with a grip extension that actually makes the gun more comfortable to hold on a compact frame.
How to Buy P320 Parts Smart
The P320 parts market has a wide price spread depending on where you shop. SIG’s own website tends to have the highest MSRP. Retailers like Brownells, Palmetto State Armory, and MidwayUSA frequently undercut SIG direct pricing by 10 to 20 percent, especially on grip modules, magazines, and small parts.
For FCUs and complete slide assemblies, SIG OEM is usually the right call. The engineering tolerances on the FCU matter, and SIG’s quality control on their own parts is excellent. For grip modules, the Wilson Combat option at $65 is better than anything SIG sells at that price point. For triggers, aftermarket (Apex or Armory Craft) beats factory every time.
Watch for P320 “parts kit” bundles that include a slide, barrel, recoil spring, and grip module as a package. These sometimes offer better value than buying each component individually. Use the price comparison grid above to check current pricing across all 80+ retailers before you buy anything.
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