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Gun Review: So Many Possibilities With the Modular P320 Fire Control Group

Sig P320 Stuff (image courtesy JWT for
Sig P320 Stuff (image courtesy JWT for

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About five years ago, the Department of Defense announced that SIG SAUER won the Army Modular Handgun contract. By the time of the announcement, there was little surprise. The P320 was reliable enough, accurate enough, relatively inexpensive, and by that time, was the only truly “modular” handgun in the running.

Since that then, SIG SAUER has doubled down, tripled down, all the downs downed on the modularity and versatility of the P320. There is now no other pistol on the market that features so many user-swappable variations, a staggering feat considering the relative youth of the handgun platform.

For the last several months, I’ve enjoyed the most aggressive Lego set on the market, shooting an extremely wide variety of slides, grip modules, barrels and small parts.

The P320 Fire Control Unit

The fire control unit (FCU) is the P320, at least as far as the law is concerned. This is the mechanism that houses the trigger and includes the rails upon which the slide is mounted. The P320 FCU is, obviously, included with each complete pistol sold. But beginning in 2020, SIG began selling the P320 FCU separately.

Sig FCU (image courtesy JWT for
SIG FCU (image courtesy JWT for

Availability is the hard part when it comes to the FCU. Demand far outstrips supply, and most of the places I called told me to expect an 8 to 12-week wait. SIG sent TTAG one to test, and I ordered another on the commercial market. It took a little over two months to arrive, and I paid about $350.

For individuals purchasing a P320 FCU from a licensed gun dealer, you’ll need to fill out an ATF Form 4473 just like you would any other firearm.

There are already multiple options for aftermarket triggers. The Apex Forward Set Trigger is a nice option that’s fairly easy to install yourself. I shot a friend’s and noticed that it did smooth the trigger out a bit and definitely shortened the reset.

A more comprehensive change is the Grayguns Adjustable Straight Trigger System. This lowers the trigger pull weight to as low as 4.0 lbs., smooths it up, and allows the user to adjust the overtravel as well. You also have your choice of trigger shoes.

This trigger is a bit more complex to install and will require you to almost completely disassemble the FCU. It’s very doable, and there are several videos online that will walk you through it, as long as you pay attention.

When it comes to disassembly and reassembly (and probably a lot of other things), I’m functionally mentally retarded. I learned long ago to take photos of every step and then simply backtrack their order for reassembly. This method has never done me wrong.

As it is, I’m happy enough with the stock SIG FCU trigger.

Grayguns Grip Module with stock trigger. (image courtesy JWT for

Since I had two Fire Control Units, I was able to measure the trigger pull of each. Using my Lyman digital trigger scale, one FCU measured an average of 5 lbs 6.2 oz over 5 pulls. The second FCU measured an average of 5bs 6.4oz.

Both of the triggers essentially feel like a mediocre 2-stage AR trigger. There’s some take-up slack, then a wall and one slight stutter before the break. Reset isn’t particularly short, but the trigger returns quickly and with a solid feel. It’s one of the better stock striker-fired triggers on the market.

The SIG SAUER P320 FCU (image courtesy JWT for

Installing the FCU is incredibly simple and requires no tools at all. You simply push it down and into place, and then work the takedown lever through the holes in the Grip Module of your choice and the FCU itself. For this review I performed this task dozens of times. I did it with my eyes closed.

P320 Slides and Slide Assemblies

Stock from SIG SAUER

SIG SAUER has released a wide range of slide assemblies, both in size and style, and the aftermarket have followed suit as well.

3.6″ Slide. (image courtesy JWT for

The factory SIG slides should best be considered by their length. Those include the 4.7″ slide (as found on the standard military M17), the 3.9 (as found on the M18), and the 3.6″ (as found on current X-Series Compact or Subcompact models.)

In addition, there’s also different styles from SIG, to include the Standard, Optics Ready, and ProCut. You can purchase the naked slide, the entire slide assembly, the assembly with your choice of sights, and also choose between the SIG Rome1Pro installed or not.

The Standard and Optics Ready are self-explanatory. The Optics Ready models come cut for the SIG Romeo1Pro or the Leupold Deltapoint Pro.

ProCut slide. (image courtesy JWT for

The ProCut slides are a little fancy, with relief cuts near the muzzle, to include a half-moon cut at the muzzle itself.

Some slides don’t fit correctly on some Grip Modules, and it’s best to check the SIG website for each slide to see what will or will not work. The general rule is that the 4.7″ and 4.9″ slides won’t go on the X-Series Compact or Subcompact Grip Modules. However, since SIG has released specialized models, like the X-Five, it’s important to check on SIG’s website for the combination of slide/Grip Module you’re trying to complete.

But wait, there’s more factory options. Each one of the above slides can be purchased with different finish options and in different colors. Want Tan/Coyote? Easy. Bead blast stainless? Not a problem. Basic Black Nitron? All day every day.

Beyond the truly comprehensive slide choices from SIG, several other companies make aftermarket slides for the SIG P320.

Grey Ghost Precision GGP320 Slide

GGP320 Slide Kit (image courtesy JWT for

I’ve never found anything less than great quality from Grey Ghost Precision, and their aftermarket P320 slide, cut from a block of 416 Stainless Steel, for the P320 is no exception.

There’s some obvious aesthetic changes with the Grey Ghost slide. The forward serrations aren’t that much different, but the single slide cuts on the side are long and relatively straight, unlike the SIG ProCut’s oblong holes. In addition, the Grey Ghost slide includes a full triangular cut through the top of the slide, ending just prior to the slide. That’s the cut that has some real value, beyond prior to just looking cool, as the dark recess serves to eliminate any glare from the top of the slide.

In addition, coming from the midline off the slide, material has been removed at two different angle toward the top, giving the slide less real estate on the top surface. This provides a bit of an optical illusion, making the entire slide seem shorter in profile when compared to the stock or ProCut factory slide.

Grey Ghost advertises their slide has “tighter tolerances” than the factory SIG slide. That’s pretty hard to measure all the way around, but I could find a difference in one spot. Placing a Backup Tactical threaded barrel and a Grayguns 14# recoil spring and custom fat stainless steel guide rod into the M17 slide, I found the barrel moved .040″ inside the slide, at the chamber. Doing the same inside the GGP320 slide, movement at this critical area was cut in half. Now, either way, that’s very little movement, and you’d likely need to be a better shot that me for that to make a difference, but there is a difference.

The GGP320 slide comes cut for the Romeo1 or Romeo1 Pro, as well as the Trijicon RMR and Leupold DeltaPoint Pro. When installing my Romeo1 Pro, I found that no mounting shim was necessary. The fit was extremely tight, with perfect (to the naked eye) mating surfaces. As we’ll see later, I had zero issues with mounting the optic or firing extended strings.

The GGP320 slide comes in several finishes, including Grey and Black Diamond Like Coating, as well as FDE and Green Cerakote.

P320 Grip Modules

SIG SAUER Stock Standard Polymer Grip Module

Since the release of the P320, SIG SAUER has released a dizzying, changing, and confusing array of grip modules for the pistol. The basic module frames include the Full, Carry, and Compact.

Stock Grip Module. (image courtesy JWT for

On top of this, there’s also the X-Series grip modules as well. Originally, SIG released the P320 with a Sub Compact grip module as well. This has now been replaced with the X-Series Compact, designed for the 3.6″ slide. So there’s a Standard Full, X-Series Full, Standard Carry, X-Series Carry, Standard Compact, and the aforementioned X-Series Compact.

The general dimensions are the same, but there are some significant differences in the grip geometry between the standard and X-Series Modules. The original standard modules have a more pronounced arch in the backstrap and that backstrap carries higher into the frame. The X-Series modules have an overall more squared grip, and the section just below the beavertail is higher and more narrow on the gun, allowing for a higher grip.

Image courtesy JWT for

The full and carry modules have the same height, and the same 17 round ammunition capacity. The Compact and X-Series Compact are shorter and require the smaller 15-round magazine to sit flush. The 17 round magazines will lock in and function with the smaller modules, they’ll just stick out a bit, as shown above. The 15-round mags won’t lock into the full and carry modules.

Small grip size, marked with an “S”. (image courtesy JWT for

Beyond the different modules, each one also comes in sizes small, medium, and large. This size refers to the diameter and shape of the grip itself. There’s not a big difference in each size.

The smallest is 140mm in diameter, the largest is 150mm, but that small amount of difference really does change how the gun feels and where your trigger finger is placed on the trigger shoe. Careful consideration can get the shooter not just the right module, but the right size for each hand to ensure comfort, speed, and precision.

Image courtesy JWT for

For instance, on any of the stock polymer modules, I prefer the large grip module. TTAG’s intrepid editor, Dan Zimmerman, prefers the medium. Dan’s current favorite P320 is a size Medium X-Series Compact module with a 3.9″ ProCut slide, as seen above.

In addition to the extremely wide array of polymer models offered by SIG, there are several variations and aftermarket options.

Grayguns Laser Sculpted Grip Module

The Grayguns grip module for the SIG SAUER P320 is a solid step up from the stock grip. There’s no real weight difference or size difference between it and the comparative length stock SIG module, and unlike the stock grip, the Grey Guns Laser Sculpted Grip Module only comes in size Medium. You can get the grip module in several colors, and the scale-like texture shows up well against the other-than-black versions.

Grayguns Grip Module (image courtesy JWT for

The big upgrade here isn’t the pretty colors or textures, it’s the shape. The wide beavertail starts high up on the backstrap, and the trigger well is undercut. Both of these combine to give the shooter a high grip on a pistol with a not very low bore axis. The texturing also covers more of the surface area of the gun over the stock version, and it seems a little grippier as well.

At $115 for basic black (shown here) and $165 for colors, this grip module is a big step up in price from the stock SIG modules, but it feels like a commensurate step up in quality as well.

Mirzon Enhanced Grip Module

Jeremy already reviewed the Mirzon Enhanced Grip Module a couple of years ago.

ProCut slide and Mirzon Grip Module (image courtesy JWT for

All I can do here is second Jeremy’s review. It’s another polymer grip, but with overmolded rubberized grip surfaces. The grip includes finger grooves that line up well with my size large hands, and, just like Jeremy, I found the Mirzon grip allows for rock-solid purchase on the gun.

There’s no twisting in the hand during long strings of fire and I had no issues even when my hands got a bit sweaty. That said, it’s a slightly more rounded grip, and shooters with small hands may need something thinner. The trigger guard isn’t as undercut as some others, and mirrors the stock P320 option.

The SIG AXG Grip Module

ProCut slide on AXG Grip Module. Image courtesy JWT for

Unlike all of the other strictly polymer grips, the SIG SAUER P320 AXG grip is all aluminum with replaceable grip panels. With a price of a penny shy of $400 on SIG’s website, it’s also a significant price departure from the inexpensive stock P320 polymer modules.

Between the wide array of grips I’ve tried with the P320 series, this one is my favorite. Of course, the first thing you’ll notice that’s different between this and all of the polymer grips is the AXG’s weight. The stock SIG polymer grips, regardless of size, weigh less than the thin paper and plastic box they come in. The AXG weighs in at 8.1oz.

That’s still not much, but compared to almost nothing at all, it’s something.

Image courtesy JWT for

Since the checkering is cut into the harder aluminum, it’s also sharper, with a traditional long diamond pattern cut into the side panels and backstrap and a 25 lines per inch hatch cut into the front strap. Compared to a stock SIG grip, there’s also a bit more surface area that’s textured.

The geometry is also slightly different than the stock SIG grip modules. The AXG is extremely similar to the Grayguns grip module in that regard. The trigger guard on the AXG module is undercut and the beavertail is both wider and begins higher up on the grip than the stock module.

When combined with the aggressive texturing of the aluminum module, the grip shape provides the shooter with a lot of very grippy surface closer to the bore axis. Recoil control, with or without gloves, is exceptional, and I never found myself fighting the gun or having to readjust my grip over long strings.

If the extra weight and all of the sharp textures buy you any more recoil management, I can’t measure it. I used a PACT timer and ran a few drills, drawing from a Mitch Rosen holster I purchased off the SIG website. Of course, being a truly modular pistol, I could keep everything exactly the same, only a changing the grip module itself.

Running the same chassis and the same slide on top of the AXG grip vs. a Grayguns grip module, I could measure no real difference in my times from draw-and-fire-two or the Mozambique/Failure to Stop drill. The only time I got any discernible difference in times was when I fired the entire magazine and even then, that was a couple of tenths of a second on average.

Image courtesy JWT for

The real advantage of the AXG grip isn’t its weight, it’s the exchangeable grip panels.  The panels allow the shooter to fine tune their grip without swapping out the module (the AXG only comes in the Carry size). For me, I like the thin G10 grip panels that come with the AXG module, but I need the large backstrap to get just the right position.

The correct length of pull will reduce fatigue and should produce tighter groups and less muzzle movement, especially during fast fire. The same test that showed little to no difference in recoil management at speed showed that my groups were, on average, just about 1/4″ smaller at 25 yards, bearing out the simple common sense that how your finger interacts with the trigger matters.


There are several other versions of the P320 grip modules available from other makers. Those include the X-Frame Legion version from SIG, modules from Wilson Combat, multiple “80%” options, and more. New versions are coming out all the time.

Small Parts

Just like everything else, all manner of parts and accessories for the P320 can be ordered directly from SIG’s website, or from a variety of manufacturers. For this review, I ordered a threaded barrel from SIG’s website and used TTAG supplied parts from Grayguns as well as barrels from Backup Tactical. I had zero issues with any of them. Everything fit and worked just fine the first time.


Mitch Rosen holster. (Image courtesy JWT for

There are more holster options for the P320 than there are slides and grip modules for the guns. I bought two always high-quality Mitch Rosen OWB leather holsters (one for the 4.7″ slide and one for the 3.9″) straight from the SIG website and used them throughout this review. They worked as flawlessly as I would expect.

Putting it all together

Since I had two Fire Control Units and a wide variety of slides, grip modules, multiple barrels and small parts, I had an absolute field day with this project. Actually, I had a few field months with it. I put at least 300 rounds through each slide (more than 500 for the GGP320), much of it suppressed. I shot various 9x19mm projectiles, commercial and hand loaded, in grains from 115 to 165, and bullet profiles to include FMJ and hollow points.

I never had any combination of slide/grip module fail with any round in any way. Nothing failed to fire. Nothing failed to load. No magazine failed to lock in place or failed to eject.

This is the reliability I’ve come to expect with all of the P320s I’ve fired, and there’s been more than a few. The SIG Romeo1 Pro optic never skipped a beat, and I mounted it on every single one of the optic-ready slides.

SIG factory X-Ray night sights. (image courtesy JWT for

When shooting off bags, untimed, at 25 yards, I found no difference in precision regardless of what grip module I used if they were the same size. Precision did change, a bit, depending on which slide I used, as well as if I was using the Romeo1 Pro optic or the supplied irons.

With the Romeo1 Pro optic and its 6MOA red dot turned way down, in slow fire with bags, and wearing my prescription glasses to correct for my eye astigmatism, at 25 yards I found very little difference in group size when shooting the same rounds through the 3.6″, 3.9″, and 4.7″ slides, regardless of manufacturer or sytle.

As long as I was using the supplied Romeo1 Pro optic, nothing I shot was smaller than 2.3″ and nothing was larger than 2.6″, on average. That’s with any round in any slide or barrel.

The only difference in precision came when I was firing the Grey Ghost Precision full-length slide with SIG irons swapped from a TTAG supplied 3.9″ slide and no optic. This combination, with Wilson Combat’s 147gr JHP bullets fired very consistent 2″ groups. That’s the best of any combination, and any round.

The real, measurable differences came into play when I was shooting multiple rounds quickly, and especially when I was trying to hit smaller targets quickly.

There was no difference if I ran the same grip module with either the 3.9″ standard or ProCut slide. Just for fun, I closed my eyes and had a friend hand swap out the standard or ProCut on the same grip module and hand it to me. I shot with my eyes closed into a berm. I couldn’t reliably tell which slide was on the gun.

As far as grip modules, the only difference I felt, at least on similarly sized grip modules, was how well I felt my hand got a solid grip on the gun during the draw. When I practiced with each grip module, there was no real difference in time, but when the weather got warmer and my hands got sweatier, the AXG model felt a little more solid on the draw.

I ran every combination of slide and grip through the Fundamentals, Accuracy, and Speed Test (FAST), well as Bill Wilson’s 5×5 test.  If you aren’t familiar with either of these tests, they’re great for measuring your overall skillset with a particular firearm, and they require relatively few rounds to give you solid feedback.

Grey Ghost GGP320 slide and Grayguns Laser-Sculpted grip module (image courtesy JWT for

I didn’t run the FAST well enough with any of the combinations to earn a challenge coin, but with either the Grayguns or AXG Grip Modul under the GGP320 slide and the Rome1 Pro optic, I got maddeningly close. So close that I burned through a month of weekly shooting drills and hundreds of rounds (six rounds at a time) in the hopes of running the test under five seconds twice in a row. No dice.

The FAST showed the real difference. I shot that test more than a second faster with the combination above than any of the other recipe. The GGP320 slide simply moved less and was easier for me to get on target and stay on target.

On the smaller slides, the 3.9″ and 3.6″, the difference in times was smaller, but it was there, and those differences were magnified in I was running the iron sights instead of the optic. More weight at the end of the barrel really makes a difference, but any increase in sight radius will likely make a more significant change than anything else.

The great thing — and what truly sets the whole P320 “ecosystem” apart — is how well all of these components mix and match, and how you can get so many options straight from the SIG website. SIG has created a very handy configurator they call the Custom Works P320 Studio that lets you pick and choose and mix and match parts — from both SIG SAUER and other aftermarket makers — until you’ve built exactly the pistol that’s right for you.

Long story short, there are far more parts and options than I’ve gone over here, and new options are constantly being released.

DZ’s preference top, JWT’s bottom. (image courtesy JWT for

For me, I found the AXG Grip module with the standard 3.9″ slide and the Romeo1 Pro works best as an EDC. But for pig hunting, I swapped the 3.9″ slide for the Grey Ghost 4.7″ slide, mounted the same optic, and ran it suppressed with an ancient AAC silencer.

If I wanted to carry while wearing a suit, the 3.6″ slide on an X-Series Compact Grip Module is small enough to boot carry…and all the with the same trigger.

With a bit of homework, and with just one part requiring an FFL and background check, anyone can create exactly the P320 they want…or create several mission-specific pistols that fit their needs, all built around that one Fire Control Unit.

A lot of companies are making “chassis” pistols, but no one else has carried through on the promise of true modular versatility like SIG SAUER has with the P320.

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