SIG P320 Series Part 3 Especially the .45 ACP!

Updated: Jul 6

Contribution from Tim Totten Stop by his Blog timtotten.com


Are you building a SIG P320 from the 80% JSD MUP 1 insert? Do you want to make sure you end up with a quality Fire Control Unit (FCU)? Are you maybe even considering chambering it in .45 ACP, but can’t seem to find any documentation on the “minor modifications” required? Well here are a few tips and lessons I’ve learned along the way. I hope you find them helpful.


The SIG SAUER P320 is an amazing, versatile, highly reliable, and very accurate pistol platform. If you’ve made your way to this post, I probably don’t need to try to convince you of that. Just consider that the P320 is now the U.S. military’s pistol of choice–in the M17 variant for the Army, and the M18 version for Marines, Air Force, Coast Guard and Navy. Even several other U.S. Government agencies (such as ICE) have selected a version of the P320 as their standard service weapon.


So now you’ve decided to try to build a P320 for yourself. Or maybe more than one? Join the club. It’s quickly becoming a very popular build. Here’s one of my early P320 builds, and still one of my go-to choices for concealed carry in many situations.

9mmOne of the author’s favorite SIG builds, a P320 X-FULL

Building a P320 80%: Just how difficult is it?


So you might ask what you are getting yourself into if you decide to build a SIG P320 from an 80% FCU. I guess there are several ways to tackle that question. If you’ve ever built a Polymer 80 Glock clone, I’d start by saying it’s slightly more involved or complex than that. You need a few more specialized tools–especially a drill press. And it takes a bit longer. But the good news is that it’s easier to get top quality results with a P320 build because there is less freehand work required, and generally fewer issues with “tolerance stacking” that often require custom fitment work with the Polymer 80s if you want the gun to run smoothly. And once you’ve built an FCU, it’s super easy to change out grip modules and slides for different configurations or calibers (with some limits).


I would also say it’s a little less work than a typical AR-15 80% build (using a modern jig). And a LOT less messy!


If you’ve never built a gun from scratch before, the P320 is a great first choice! You just need to spend some time getting the right equipment and setting it up properly, take your time, and you’ll be fine.


Speaking of time, you’ve probably seen all those hyped up media stories, or the press releases from certain politically motivated officials, about how an evil “ghost gun” can be built from scratch in 11 minutes using a butter knife, fingernail clippers and some duct tape. Even some of the 80% manufacturers–stupidly, in my opinion–over-sell how fast you can complete a build. Just forget all that


I mean if my goal was to win some kind of race to build something capable of firing off a single shot for some illicit purpose, and I wasn’t worried at all about quality, workmanship or safety, then yeah, I could probably just skip the whole 80% parts market and whip up something in a few minutes with some common parts from the local hardware store. Why aren’t the over-regulators going after that?!


But if you want a beautiful, quality pistol that is as safe as a factory build, cycles like butter, shoots accurately (perhaps more accurately than an off-the-shelf model), and runs reliably for thousands upon thousands of rounds, it’s going to take a little time and a commitment to doing things properly. Just gathering all the needed tools–and especially the required parts–will take a few weeks in this crazy environment (early 2021, as I write this).


For your first build, once you have gathered all the parts and tools and fully set up your work area, I would plan on at least 6-8 hours of work–preferably spread over a weekend or a couple of evenings–just to get the basic build working. Then you need a little time on the range to get the sights dialed in, as well as the ejector on the .45 ACP model (more on that later). And if you want to add a red dot, or an adjustable trigger, or some other fancy stuff, that all takes additional time. Like anything else, once you’ve done it a few times, you’ll get a little more proficient.

The author’s P320 X-FIVE build

Where can I learn how to build a P320 80%?

You’ve probably seen all the videos and web pages that are dedicated to 80% builds for Polymer 80 and other Glock clones, or the AR-15 platform. Because the platform is newer, and the 80% build options haven’t been available for very long, there is not quite as much info out there on the P320 as there is for some of those other platforms. Especially lacking is the information you need to know if you want to build a P320 chambered in the venerable .45 ACP


By the way, for Glock builds, see Marine Gun Builder. He’s the best. No one else is even close. First Time Quality! MGB is currently working on a series of videos for the P320 platform, and I suspect those will largely obsolete what I’m publishing here once they’re released. He’s definitely the pro. I’m just a random guy on the Internet, trying to fill a gap in information in the meantime.


You definitely want to sign up for the forum on MGB. There’s a sub-page there dedicated to SIG SAUER builds. You’ll often find me posting and commenting there. It’s a great place to ask questions and share information.


In the meantime, while we’re all waiting for MGB to post his upcoming P320 video series, probably the best source for videos on P320 builds is The Rogue Banshee. Jason also does some interesting work on several other firearms platforms. Of course, you’ll also want to watch the brief official MUP 1 instructions video from JSD at least a time or two before starting your project.


What tools do I need to build a P320 from an 80% module?

MGB has already posted a list of tools for P320 builds in his forum. You’ll have to subscribe to the forum to see it. I’ll defer mostly to that list, with a few exceptions and additions noted here.


MGB recommends the WEN 4214 12-inch drill press, which runs a bit over $400. You can actually complete a P320 build just fine with the much less expensive WEN 4208 8-inch drill press. That’s what I personally use, and what I recommend. It’s less than one-fourth the price ($87.37, as I type this in early 2021). You’ll just need to use one little trick, which I’ll explain below.


MGB recommends the WEN 424DPV drill press vise. But I definitely suggest spending about $20 more to get a nice X-Y milling work table. While it takes a little customization to fit this table onto your WEN 4208 drill press (trust me–you can figure it out!) having the ability to fine-tune X and Y adjustments will really save you a lot of time and make it easier to achieve quality, accurate drilling. And with the P320, there are lots of holes you need to drill accurately.


This X-Y table is definitely not a high-end, super precision product. If you can afford such a thing, great. But this one is perfectly acceptable as long as you take care to tighten up the slack in the adjustment screws from time to time. If you don’t, it will definitely loosen up over time and start to wobble as you drill. That’s not good. So check this before starting to drill.


The author’s X-Y table, mounted on the WEN 4208. Note that the highlighted adjustment screws must be tightened from time to time to prevent excessive wobble when drilling.

Forget the drill bits that come with the JSD P320 jig. If you’re super slow and careful, they might work for one build. But they won’t last long, and chances are that you’ll probably burn up a bit or two before you finish your first build. So just throw them in the trash and get some quality cobalt bits, as recommended by MGB. I’ve used the same set of cobalt bits for nearly a dozen P320 builds now.


Now, one of the issues you will run into is that there is not quite enough clearance once you mount the X-Y milling work table to the WEN 4208 for a standard-length 9 mm bit to clear the JSD jig. But don’t worry–the workaround is simply to rotate the drill press work table to one side, push the jig up onto the bit, then rotate the work table into position. The bit will end up slightly inserted into the jig, but not quite touching the MUP 1 insert. This is fine as long as you carefully align the hole before you start drilling. You’ll only need to do this for the two 9 mm holes. All other bit changes will work out fine without having to reconfigure your setup.


In addition to MGB’s tool list, I suggest adding a narrow cut 4 file that has two safe sides, and if you don’t already have one, a Nicholson universal file handle to go with it. This file is really handy for getting into tight places without damaging sections you don’t want to mar. (Don’t tell anyone, but I also use this file sometimes on Polymer 80 tabs, right after the fret cutter does 95% of the work.)


I used to swear by the Grayguns P320 Armorer’s Block, but after MGB introduced us to The SIG Armorer’s Block, I’ve been using that quite a bit more. It’s definitely worth the investment.


Finally, MGB recommends Forney Tap Magic cutting fluid, and I’m sure that works great. But I’ve been using the Morris Products (Oatey) G30203 Dark Cutting Oil with great success. And it’s a lot cheaper.


What are the parts I need to complete a SIG P320 build?

For a typical P320 build, chambered in 9 mm, .40 S&W or .357 SIG, see the tables below for the basic parts you will need to acquire. Obviously, you will need the appropriate magazines, and you might want to add options like a red dot optic or light. And note that this list does not include the parts for a manual safety. (I’ll admit I’ve never actually built a P320 with a manual safety.)


The parts listed as “multiple” are where you can do most of your customization, either by choosing aftermarket parts, or sometimes even multiple offerings directly from SIG SAUER (e.g., you might prefer the SIG flat trigger vs. their more traditional curved trigger). Reference the SIG P320 schematic on SIG’s official P320 Small Parts Shopper page (which is one option for acquiring parts, although you can generally find better pricing elsewhere).


A few random tips for building your SIG SAUER P320


Here are a few lessons I’ve learned, mostly the hard way.