Window of Opportunity: The Modular Pistol Revolution

To begin, I should mention that our main carry pistols here at the ranch are all Glock Model 21 and Glock Model 30 .45 ACPs. I intend to keep those as our family’s standard for the foreseeable future. But in the past two years I’ve branched out into buying several SIG Model P320 9mm pistols. I like the clever modularity of these pistols. The same serialized trigger group module can be used in assembling everything from a subcompact pistol to a full size race gun. I also find their ergonomics quite good, their controls instinctive, and their trigger pulls decent–right out of the box.  Notably, a subcompact variant of the SIG P320 is now my designated “Visiting the Big City” carry pistol. My willingness to carry that pistol on those infrequent trips tells you how much trust the design.

SOme History

I should back up a bit and explain that there are have been four major waves in in the history of semi-auto pistols: The first wave began experimentally before 1900 with some rather clunky steel frame designs like the C93 Borchardt and the Mauser C96 “Broomhandle”. The second wave was typified by the sleek Browning design, with many variants that dominated the market for the next 80 years. The third wave came with the advent of the Glock 17, which was the first practical and popular striker-fired polymer frame pistol design. (Although, arguably, it was predated by a decade by the HK VP70. But that was a design with both atrocious sights and a very heavy, creepy trigger pull. Thus, Glock was the first company to Get It Right, with a polymer frame pistol.) The fourth and most recent wave of handgun design combines both polymer frames and modularity.

The New Window Design

The key distinguishing feature of many modular pistols is that the pistol’s polymer grip is not legally considered the “frame.” Rather, that is just the small removable trigger group that caries the serial number.  The serial number can be seen through a slot or “window” molded or milled into the grip. Thus, when ordering parts to build or modify a modular pistol, under U.S. Federal law you can buy a variety of barrels, slides, magazines, and grips without any restrictions. It is only the serialized trigger group module that legally constitutes the “firearm”.

SIG USA sells parts sets for the P320 as what they call Caliber XChange Kits. These allow existing owners to drop their trigger group module into a new full set of parts in a different caliber or configuration. The kits include everything except a trigger group.  So, for example, your trigger group module could be moved from a .40 S&W subcompact into a full size 9mm parts set, without making another firearm purchase. Again, it is just the trigger group that carries the serial number and that is the “firearm.”

The first modular pistol that I owned was a SIG P250. (A model that was discontinued in early 2017.) This, as I recall, was the first patented modular design to incorporate a removable serialized trigger group module and an unserialized grip frame module with a window/slot for observing the serial number. The P250 design was essentially a transitional “proof of concept” for SIG. It has now been fully supplanted by the SIG P320. Thankfully, the 9mm P320 uses the same magazines as the P250, so early adopters didn’t have to buy all new magazines when transitioning to the SIG P320.

In 2017, the U.S. Army announced that two SIG P320 variants had been formally adopted as the replacements for the full size Beretta M9 (Model 92) and the compact SIG M11 (Model P228). The M17 replaced the M9 and the M18 replaced the M11. These are much like their civilian counterparts, but they both have an external thumb safety.

The Drop Problem

Soon after the military adoption, there was an alarming revelation: The SIG P320 could accidentally fire if it were dropped and the rear of the slide hit a hard surface. Needless to say, this was a public relations nightmare for SIG USA. Coincidentally, it was found that SIG P250s did not have the same weakness.

SIG’s engineers did their own tests confirmed the problem and began a rapid redesign of the slide and trigger group. Within a few months, they announced the release of a new “drop safe” P320 design.  Rather than issuing a full blanket recall, SIG chose to announce a “voluntary upgrade” program for all owners of existing SIG P320s. To their credit, they cover 100% of the expense of the recall, including the cost of shipping, both ways. Most dealers, police departments, and individual owners have now had the Drop Safe upgrade work performed on their guns. And all of the U.S. production is of course with the corrected design. But do I believe that SIG USA should have made this a full mandatory recall.

When shopping for used SIG P320 be aware that there are still lots of older P320s in circulation that have had not yet had the Drop Safe upgrade work done. To be certain, there are several YouTube videos that show the difference that is visible, when you lock the pistol’s slide to the rear. In my own experience, I’ve found that the upgrade process only takes about three weeks. So I don’t have any reluctance in buying a complete “pre-upgrade” SIG P320 or a separate trigger module. In fact, these can often be bought at a slight discount. If you do own a P320 that hasn’t been upgraded, then I highly recommend that you go ahead and have that work done. Again, it costs nothing but a bit of your time. The end results will be peace of mind, a slightly lighter trigger pull without the annoying quiet “click” halfway through the trigger pull, and a pistol with better resale value. SIG USA also encloses a nice  “25% off” coupon for your next order from their web store.

Some Modular Competitors

Now that the much-publicized drop safety issue has been resolved, I predict that the SIG P320 family of pistols will be more popular than ever. The Beretta APX is another great modular design, but it didn’t win the Army adoption trials. (Apparently, SIG won the contract award because their design was comparable to the Beretta APX, but their “cost per pistol” bid was substantially lower.) The APX mimics the SIGP250/P320 “serial number window” design. Another competitor in the Modular Handgun trials was the FN 509. But it–along with the Glock trials entries–carries the serial number on the forward portion of the grip frame. I expect that the Beretta and the FN will also be quite popular, if they remain price competitive. But the SIG P320 will probably gain market share because of the cachet of being the Army’s standard pistol for many years to come.

Meanwhile, the popularity of the Glock 17/19 family is very well established, and civilian sales of the SIG P320 variants may never catch up with its Glock equivalents. The management of Glock was very disappointed with their entry’s failure to win the Modular Handgun trials, and filed a formal protest.  But the Army turned that down and proceeded with their buy of the SIG M17 and M18.

I can anticipate that SIG will eventually branch out with even more caliber and design options for the P320 family of pistols. Offering .22LR XChange kits would be a natural progression. I also expect them to offer a wider range of grip frame modules with various grip angles, gripping surfaces, lights, and lasers. Because the .45 ACP variants and the  9mm/.40 use different trigger group modules that cannot be interchanged, they are essentially two major branches of the same family. Presumably the .45 ACP branch could spawn a 10mm variant.

Endless Possibilities

All in all, the SIG P320 is a game changer for the world’s firearms market. I expect many other manufacturers of pistol caliber guns to follow suit. Even some rifle makers may use serialized trigger groups and a window. Most European countries put purchase restrictions on firearms barrels (and other pressurized parts). But here in the States, we can buy any gun part other than a serialized frame, receiver, or trigger groups without any “Mother May I” paperwork.

Just the first of many SIG P320 spin-offs is called a Fire Control Unit. It is a clamshell housing that outwardly resembles a submachinegun, but it is legally just an arm-braced pistol. I’ve been experimenting with one of these for just a few weeks. For safety, since this gun lacks an external safety, it should be carried “Israeli style”–with an empty chamber and racked only just before firing.

The new era of firearms modularity has opened up all kinds of opportunities for gun hobbyists. Speaking of which…

A Pet Project — Jim Needs Some 3D Printer Time

On a related personal note, I have several design modification ideas for SIG P320 grip frame modules in mind. So I’d like to rent some time on a high temperature 3D printer capable of laying down polycarbonate to build some prototypes.

If you have such a machine available–or if you know of someone who does and they are pro-gun–please let me know. I’d be renting time on a 3D printer to create a few custom SIG P320 grip frame modules. Again, this must be a high temperature 3D printer capable of handling polycarbonate. (Lower melting point plastics won’t work for this high stress application.) My #1 Son will provide the completed CAD file. I’ll provide a full roll of green or black polycarbonate filament.

I’d prefer to swap, but I could also pay cash for the 3D printer time and the mailing expense for getting the completed grip frame modules back to me. (Note: Since this project won’t require any serialized trigger group modules on site, the grip frame modules can go through the mail with no FFL required. Nor does the owner of the 3D printer even need to have an FFL.)

Here are some possible trades, in exchange for the 3D printer time:

  • Some completed SIG P320 grips, for your personal use. (But I would retain the design copyright.)
  • Original Steyr AUG 42-round magazines
  • AGP brand 8-round magazines for Saiga 12 shotguns
  • Original factory Beretta M92/M9 30-round magazines
  • Autographed copies of most of the books that I’ve authored
  • HK G3 20 rd. magazines–both alloy and steel available

Many Thanks,



  1. Interesting article and some ideas, but ,being an old fart and stuck in my ways. I still prefer my 1911. But I have moved on in one way, I recently picked up a S.F Armory compact in a 9 mm. It really rides so nice IWB.

  2. Jim, Thanks for the heads up on the P320 Sig. My number one son has one in his rather large (huge) gun safe I don’t know how I remembered that he had it:) and we were unaware of the voluntary recall for repair so I text him the info along with your link to Sig.

  3. How long before some one starts selling rifles which take the Trigger Modular after all the mag does not have to fit into the Trigger Modular.
    If some one wants to get into the business of making guns but does not want all the paper work this is the way to do it.
    How about a lower which takes AR 15/10 uppers? Or a bolt action, may be a shotgun in fact it could be anything.

  4. For $99 Sig also sells a P320 air pistol. They are great for training – same weight, same trigger and the slide moves to disturb your sight picture. If 9mm is $0.15 a practice round I will have saved about $1,500 by the end of the year.

  5. A couple of thoughts.

    I would start the 2nd wave of automatics with the M1911 introducing ergonomic and practical pistols, and the third wave with the P38 as the first effective double action full power pistol.

    I think the VP70 gets a bad shake for its trigger. I have one, and yes, its trigger pull is different, but I don’t find it that bad. The sights work in the conditions I have shot it so far, but there may be lighting conditions I haven’t tried where they could be a challenge.

    Finally, I did buy a used SIG p320, and so far like it, although I have only put about 150 rounds through it. I do question the idea of buying accessories to change the frame size, barrel length, etc vs buying another gun. Mine cost $405, which is less than some of the caliber exchange kits I have seen.

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