Everything You Need to Know About the P320: Exeter, Aftermarket Parts and Beyond

The Sig Sauer P320 handgun is modular by design. The ability to swap calibers with different sized frames and slides provides a versatility that is driving a massive aftermarket for parts and accessories. Building a P320 is the perfect way to get exactly the gun you want without having to buy a new gun and throw away parts.

I have quite a bit of experience with the P320 now and I have collected some of the best aftermarket parts. I can’t talk about everything because new parts pop up every day. I wanted to share some of the things that I have found to work and my optimism about the P320 system.

Sig Sauer has made over 1 million P320 pistols and demand continues to exceed production. This modular wonder, made in Exeter New Hampshire, is perhaps the most tested handgun ever. The P320 has been selected by all branches of the U.S. military, the Department of Homeland Security, and many other military and law enforcement agencies around the world. What makes the P320 so special?

The P320 FCG is the serialized firearm as opposed to the other handguns where the frame carries the serial number. This FCG has been upgraded with the APEX trigger and Trigger Bar. Original OEM parts are shown below.

The unique feature of the P320 is the Fire Control Group (FCG), a self-contained drop-in trigger box. The FCG can be dropped into subcompact, compact, carry or full-size kits including grip module, slide, barrel, recoil spring, and one magazine in 9mm, .357 Sig, .40 S&W, or .45 ACP.

The P320 modular nature has provided a massive market for upgrading and tuning this gun. SIG’s standard grip modules are available in small, medium, and large sizes. This refers to the circumference of the grip only and is analogous to different backstrap sizes on other pistols. The beginning of customization is the grip module. It is easy to switch out and change everything.

SIG has its own aftermarket products to upgrade your stock guns. The new Pro-Cut slide has aggressive styling and accepts ROMEO1Pro footprint optics. The X-Carry grip module is sleek and functional.

I started out as a skeptic on the P320. As I watched the military and Homeland Security adopt it, I realized SIG must be on to something. I bought a used .40 just to see what the P320 was all about. I soon became a fan.

The military wanted a modular system and SIG had the only candidate that filled the bill. I have played with Glocks and other guns, changing slides and barrels, sights, and triggers but this is different.

The Fire Control Group lets you change everything around it. Changing grips and calibers are easy, like stupid easy. As more and more companies step into the 320 market, we are on the verge of a SIG less 320 with no OEM parts.

You might think this is bad for SIG, I disagree. As the 320 becomes the standard for the 21st century, SIG’s sales of guns and parts will increase with the demand for aftermarket parts.

With a new factory in Oregon, Sig has positioned itself as a leader in handgun optics. The Romeo1Pro is a big step forward and the Romeo2 is a substantial duty sight. The police market is ripe for this and no other gun company can ship a gun with their own optic on it.

SIG makes the ultimate accessory, the caliber X-change kit. Take your FCG and drop it into a whole new gun. They also offer optic cut slides and Pro-cut slides with an aggressive custom look.

The military’s Modular Handgun System has two versions. The full-sized model known as the M-17 and the carry-sized model the M-18. The P320 selected by DHS is a version of the X Carry.

To decide what your P320 should look like, start with the Standard P320 Configurations:

P320 Full-Size

Length: 8.0”

Barrel length: 4.7”

Width: 1.4”

Height: 5.5”

Weight: 29.4 Oz

P320 Compact

Compact grip module is optimized for 15 round flush fit mags.

Length: 7”

Barrel length: 3.9”

Width: 1.4”

Height: 5.3”

Weight: 25.3 Oz

P320 Carry

Carry models accomodate 17 round mags flush.

Length: 7.4”

Barrel length: 3.9”

Width: 1.4”

Height: 5.5”

Weight: 27.0 Oz

P320 Subcompact

Subcompact grip module is optimized for 12 round flush fit mags.

Length: 6.7”

Barrel length: 3.6”

Width: 1.3”

Height: 4.7”

Weight: 25.0 Oz

Converting your P320 from one configuration or caliber to another is as simple as swapping out the FCU. Each version is essentially an entirely new gun, but legally, it’s the same firearm because the Fire Control Unit is the serial numbered part which is the gun. This provides a unique opportunity for 80% builders.

JSDsupply.com makes a P320 80% Fire Control Unit called the Modular Universal Pistol-1. Made from 316 stainless steel, the finished MUP-1 has the same dimensions and specs as a factory FCG. The MUP1 is compatible with the P320’s drop-safe upgraded parts and all calibers (.45 ACP requires slight modification).

While building a gun at home is legal under federal law, some states – like California and New York – have severely restricted or banned gun-making kits like this, so you’ll need to double-check your local and state laws before starting this project. Your freedom may vary.

The MUP1 Jig sets you up to drill the right holes, open up the trigger slot, and bend the slide rails. I found the Rogue Banshee’s videos very helpful.

I didn’t have variable speed on my drill press and had some problems with the drill bits crushing the frame instead of cutting cleanly. I broke a wood paint stirrer into sections and used it to shore the inside of the frame.

You do need some basic metalwork skills to do this. My next one will be better. Check out the quick video to see how the jig works.

Once your FCU insert is machined, you can install the P320’s lower parts kit. This can be confusing and expensive if you try to find the individual parts yourself. You need to get the upgraded parts to work with the MUP1. I talked with the guys at SmallGunsParts.com and got everything I needed. They have SIG OEM parts.

Assembling SmallGunParts into a complete FCG is pretty simple.

At this point, you technically own a functional P320! A simple option to finish your pistol is to buy a P320 Build Kit direct from SIG Sauer. Just pick your configuration – full-size, carry, compact, or subcompact. All the available build kits include the grip module (frame), slide, barrel, operating rod and recoil spring, magazine, and preinstalled sights. There are models that include red dot optics also.

Built on an actual SIG-SAUER P-320 Frame, Smart Firearms SF-35 Pro-X Training Gun is feature rich at the pinnacle of evolution for training guns.
Round Counter – forces a reload after the designated number of rounds are fired.
INTERNAL LASER – allows you to see your hits
Rumble Motor allows sensory perception that the gun is firing.
USB Rechargeable
Night Sights
Removable Magazine

Grip Modules

One of the best features of the Sig Sauer P320 is the ability to change the size of the gun, caliber, and the entire look of your weapon. Companies like Small Guns Parts offer you a variety of different color modules like grey, green, bronze, and more. You can order an entirely new gun, minus the FCG without an FFL.

Icarus Precision Ace 320 Grip

The Ace is available in five colors: Black, Flat Dark Earth, Midnight Bronze, OD Green, and Sniper Gray. If you want a custom color, they also sell the Ace in Raw Aluminum so you can paint it.

Want to upgrade the polymer grip? You can go completely custom with the 7075 T-6 billet aluminum Ace 320 Grip. The Ace works with all OEM slides 6.5 inches or longer. The weight and features of the Ace promote good recoil management and positive manipulations.

An extended beaver tail raises the grip position to improve ergonomics to land perfectly in the web of your hand reducing muzzle flip. The exaggerated finger notch below trigger guard reduces front to back grip width and improves access to the controls. The unique grip texture uses alternating circular pocket design offering excellent grip and a natural shooting position. 

Amend2 Magazines has announced the S300, a modular pistol frame designed for use with the P320™ Fire Control Unit and Slide. The S300 uses P365™ magazines to reduce the width and length of the frame, making this an ideal concealed carry package.

Grey Ghost Precision Slide

The P320 accepts different slide lengths. You can build the exact gun you want with all the desirable features. Grey Ghost Precision GGP320 slides are machined out of 416 stainless steel. They are compatible with SIG P320 Full/Carry and P320 XFIVE frames.

You don’t have to let your choice in miniature red dot optic be driven by mounts that accept a single footprint. The most innovative feature of the GGP320 slide is the hybrid MRDS optic cut which allows mounting several handgun reflex sights: SIG Sauer ROMEO1, ROMEO1Pro, Trijicon RMR or Leupold DeltaPoint. The screws and shim plates necessary to install the mentioned sights as well as with a G10 cover plate for the optics cut are included in the package.

The GGP320 slides offer enhanced aesthetics, increased grip serrations, and an improved coating without compromising function. Romeo1 sight, SmallGunParts grip module, and Kres trigger.

The GGP320 slides offer enhanced aesthetics, increased grip serrations, and an improved coating without compromising function. This stripped slide is machined from 416 Stainless Steel for tighter-than-factory-tolerances.

There are lightening cuts at the top and sides of the front portion of the slide as well as enhanced serrations. For corrosive and abrasion resistance, these slides are available in black and grey DLC or FDE Cerakote finish matching the Coyote Tan Grip Modules used in the M17 and M18 issued pistols. The GGP320 is a drop-in install with the enhancements that professional handgun shooters demand.

Faxon Barrels

The barrel is the heart of any gun. It is the source of accuracy and can provide a unique look. Faxon barrels are a perfect upgrade for any P320.

The barrels are available in different lengths, but also a multitude of different finish options including TiN, Rainbow, and black. The barrels are fluted in Faxon’s flame fluted pattern.

The P320 has great accuracy potential. The Faxon barrel lived up to my expectations. These typical groups were shot standing unsupported with a ROMEO1.

Faxon barrels are made from 416-R stainless steel and have an 11-degree target crown, which ensures equitable gas relief and will add that hair of perfection to your gun’s precision. The inside finish of these barrels is a strong salt bath nitride finish, and the outside is TiN.

Faxon makes threaded barrels with standard ½ x 28 threads specially made muzzle device making it easy to add a suppressor or a compensator. The GSL Stealth suppressor is a perfect match.

Apex Trigger

The trigger is your primary interface with the gun. P320 triggers are known to be pretty good. I’m a big fan of the flat triggers, and I find it a lot easier to reach and pull consistently.

A controllable and consistent trigger will help you shoot better and shoot faster. The Apex Forward Set Trigger Bar is a billet steel drop-in replacement to the factory trigger bar and is easily installed without the need for tools or fixtures. It reduces trigger travel and reset approximately 20% and pull weight by 10%.

Apex produces flat-faced, curved, and semi-flat triggers to get you exactly what you want. The Apex Trigger Bar can be paired with the factory curved trigger or one of the three Apex triggers for the P320.

Keres Pro Adjust Trigger

The Keres Dynamics’ PRO ADJUST trigger allows you to adjust both trigger pre-travel (take-up) and over travel. The flat design features a center rib and curved tip to provide tactile feedback for fast and consistent indexing.

Overall trigger travel can be reduced by up to 60%, providing a short crisp trigger pull and reset. The increased leverage reduces the trigger pull by 25%.

Keres Dynamics’ PRO ADJUST trigger and Keres Pro Extended Magazine Release

Keres Pro Extended Magazine Release

Keres also makes the PRO EXTEND magazine release to improve magazine release access and indexing, allowing for faster magazine changes with enhanced ergonomics and comfort.

XS Sights

Sights are the other critical part of the shooter interface. P320 sights on this pistol are standard and interchangeable with other SIG models. There are lots of options from the aftermarket. I decided to go with XS’ DXT2 Big Dot Yellow sights on the Grey Ghost Precision sights.

DXT2 Big Dot sights drive focus to the front sight using a high-contrast front and a V-notch rear for an unobstructed view of the front sight. This pairs perfectly as a backup sight with the ROMEO1’s built-in rear notch.

The Big Dot front sights use a glow dot that absorbs light and glows in low light levels before it is dark enough to see the tritium. Optic Yellow is the best option for low light and has the brightest glow intensity. The increased surface area of the convex dot increases light absorption, intensifying the charge of the glow properties of the dot.

Sig Sauer ROMEO1 PRO Miniature Red Dot Sight

Shooting with a mini red dot takes some practice and training to get right. It’s faster to get on target, faster to transition, and easy to use. The red dot lets you focus on the threat. No need to line up your sights, put the dot on the target, pull the trigger, and go.

With aftermarket slides, you have options. I decided to mount a ROMEO1 optic on the Grey Ghost Precision slide. The ROMEO1 has an optional cover providing increased ruggedness.

ROMEO1 is a feature rich yet affordable miniature red dot sight. The motion-activated illumination system powers up when it’s moved and powers down when it’s left stationary. MOTAC provides readiness without excessive battery consumption.

Safariland 6360 RDS Duty Holster

Safariland is famous for their quality duty holsters. 6360 isn’t the most descriptive name for a holster, but it’s an excellent option for those looking for a duty retention rig which accommodates a red dot sight and a light with retention.

The ALS/SLS retention system is intuitive allowing you to draw unimpeded in a fluid motion. As the weapon is holstered, it automatically clicks back into retention eliminating worry about the gun falling out.

Safariland 6360RDS holsters have both the ALS/SLS retention. It accommodates a Streamlight and an RDS. The TLR-VIR provides a White LED Weapon Light with Infrared LED/Laser.

The Safariland 6360RDS is all polymer and compatible with a wide variety of attachment systems to carry the holster in a variety of ways, including thigh rigs, as well as low ride rigs, belt rigs, and even MOLLE setups.

Safariland’s VUBL Vertical Belt Loop allows the user to quickly raise or lower their holster without any tools required. The VUBL passes the Retention™ pull test and locks onto any duty belt with a belt slot design.
A wide variety of concealment holsters are available. Quality custom holsters like this ANR Design model provide safety and retention. Photo by Zach Verock.

Streamlight TLR-7A

A weapon light can provide critical situational awareness for professional use or home defense. Without threat identification, you can’t take a shot.

The TLR-7 is an ultra-compact weapon light that sits flush with the rail and the barrel of the P320 Compact and X-Carry. The TLR-7 is ultra-lightweight, easy to use, and perfect for concealed carry. The Compact grip modules makes a small functional package.

Ultra-lightweight & compact, the TLR-7A Tactical Light uses LED technology for 500-lumen output and provides 1.5 hours of continuous runtime from a single lithium battery. The TLR-7®A model has rear switches for intuitive ergonomics.

500-lumen output delivers a powerful beam with an extensive range, good peripheral coverage, and selectable strobe mode. Rail clamp designed to rapidly attach/detach from the side of the weapon.

I see a bright future for the P320, not just because of what it is, but because of what it will become. As designers step forward to create clever new parts, the design will mutate and remain forever relevant. 3D printing will accelerate this. I can’t wait to see the next thing.

About the author: Mark Miller is a former Customs Agent and a Green Beret who served in Afghanistan and a number of other live fire locations. A student of firearms and shooting, he is an FFL and a SOT. The guiding philosophy of his life is that terrain and situation dictate tactics and the enemy always gets a vote on any plan.

{ 15 comments… add one }
  • Mikhail Green September 14, 2021, 12:50 pm

    Can anyone help me out? I’m trying to put a compact slide and barrel on a full size Grip. I’ve seen it done in numerous pictures online but it’s not working for me. Slide won’t go back far enough (2mm) to engage the take down lever. Killer innovations compact slide and barrel on a full size wilson combat grip.

  • Dan A-F November 7, 2020, 7:41 am

    Mike, thanks for this write up. I’ve got a p320 compact and love the fit and feel of it.

    However, I am concerned about the rising number of accidental discharges, unrelated to drop testing, sometimes even from the drawing of the pistol. Sig claims that their voluntary recall fixed many issues, but there have been local issues of a handful of local LEOs being injured by their own firearms, even after the fact. Sig has maintained the innocence of their pistol, but I find myself reticent to carry it as an every day carry, which defeats the purpose of the pistol for me.

    Do you have any inclination or clue as to what the cause of these issues are and if any kind of aftermarket parts kit may be suggested as a remedy for it? I am afraid that the cause is in the design and not in any quality of the pistol manufacture, and thus unlikely to be solved by another fcg.

  • Bill S July 4, 2020, 9:39 am

    When the P250 and then the P320 came out, I told everyone I knew that this was going to be the pistol equivalent of the Ruger 10-22. It took awhile for the after market parts manufacturers to bring their enhancements to market. I don’t think we’ve seen the end of the possibilities yet.

  • Tom D July 1, 2020, 6:39 am

    No one has lower parts kits for the P320s. Maybe you got yours some time ago from the site you mention. Let me know if you see otherwise.

    • Mark Miller July 4, 2020, 11:41 pm

      FCG parts go in and out of stock. I5 Guns in Olympia WA is working on a source for complete FCG’s.

  • Dan V. June 30, 2020, 9:21 am

    The compact and carry variations are not the same. While they do share the same slide/barrel lenght (interchangeably), the compact uses a smaller grip module designed for the 15 round flush fit mags. Carry models have what is essentially the compact slide lenght on the full size grip which fits 17 round mags flush.

    • Mark Miller June 30, 2020, 10:17 am

      Thanks for the help Dan.

  • Cary June 29, 2020, 3:08 pm

    I have 6 320’s now in various sizes and calibers. I love them. This platform will likely get me to the end of my days. Only thing I ever do is stipple the grip myself a bit more. My dept is going to the 320 X in 9mm. We are currently issued Smith 45’s. I like the Smith fine but I love the 320s.

  • David C June 29, 2020, 10:38 am

    Polymer80 has released their Sig modular grip assembly. I like it a lot better than the either of the Sig grips I’ve tried.
    Unfortunately, the mag release is plastic so my factory metal mags have cut the magazine catch off after about 500 rounds worth of mag changes.

    • Mark Miller June 29, 2020, 11:47 am

      Can you replace the Polymer80 magazine release with SIG OEM parts?

  • chinchy bugg June 29, 2020, 9:14 am

    Another thing you need to know: the Romeo 3 Max is marketed by Sig as a sight for “…large frame pistols”. It doesn’t fit on a P320 Legion. How much bigger does a gun need to be to make Sig’s claim truthful?
    Thanks Sig marketing and (non)customer support. I love making $600 mistakes.

    • Mark Miller June 29, 2020, 11:50 am

      Direct handgun slide mounting options for the ROMEO3 MAX are currently limited to the SIG P320 X-Five with a Springer Precision Romeo3 adapter plate. Other adapter plates are on the way, keep an eye out for a plate that will work for your P320 Legion. The Picatinny mount allows it to be easily attached to any rail, which makes it a good choice for rimfire pistols and rifles, PCC or Open Class pistols.

      • chinchy bugg June 29, 2020, 2:19 pm

        Thanks Mark. Sig should hire you to fix their careless product description, assuming they are at all concerned about people spending top dollar on incompatible equipment. Base on my experience though, getting my money is their first and foremost objective.

  • Brad S. June 29, 2020, 9:08 am

    Great article. Well written and very informative. Yet to run a P320, but this article has me on the doorstep, wallet in hand.

    • Mark Miller June 29, 2020, 11:51 am

      Thanks Brad, I am really enjoying the new options for the P320. It just gets better and better.

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