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 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.
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:
Barrel length: 4.7”
Weight: 29.4 Oz
Compact grip module is optimized for 15 round flush fit mags.
Barrel length: 3.9”
Weight: 25.3 Oz
Carry models accomodate 17 round mags flush.
Barrel length: 3.9”
Weight: 27.0 Oz
Subcompact grip module is optimized for 12 round flush fit mags.
Barrel length: 3.6”
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.
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.
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.
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.
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. 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.
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.
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.
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.
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 also makes the PRO EXTEND magazine release to improve magazine release access and indexing, allowing for faster magazine changes with enhanced ergonomics and comfort.
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.
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.
A weapon light can provide critical situational awareness for professional use or home defense. Without threat identification, you can’t take a shot.
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.