P320 80% Build Adventure article post image

P320 80% Build Adventure

So you are thinking about building a Sig P320 80% pistol. Awesome, you have come to the right place. I am going to walk you through the adventures that I had when making the Sig P320 80% video.

I am not big on the unboxing videos and quite frankly, had no intention of writing this article. However, I did run into some minor issues early on this build which prompted me to write this.

Because there was no intention of writing this article, I missed the opportunity to get pictures of the jig when I took it out of the box. 


Before I go any further, I want you to understand that this is an unbiased article. There was no vendor solicitation and everything here was purchased at full cost. These vendors have no idea that this article is even being written and I did not tell them that I was an FFL holder (not that makes a whole lot of difference in the 80% world). 

For full disclaimer, I am a Type 1 FFL which allows me to sell firearms and do gunsmithing activities. It does not allow me to manufacture a firearm for resale. All 80% firearms that I build are for personal use only and are not for sale. Some fixtures, tools and/or jigs that I may talk about or show in a video series may be owned personally and not by my company (Eagle Eye Shooters Supply). 

I do not advocate building any firearm or suppressor to bypass (break) Federal, State or Local laws or ordinances.

Since the video series went live, Jordan from JSD Supply asked if I would like to become an affiliate which I accepted. Links to JSD Supplies’ parts and tools on this website are now affiliate links. If you like the work that we did on the video series, please consider clicking on our affiliate links when purchasing parts or tools.

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The Story

There is no doubt that it is much cheaper to buy an off the shelf firearm than build one and 80% builds can be more expensive than other custom built guns. It is important to remember that 80% builds are typically done for the pride of accomplishment, as well as, having a totally custom gun that meets the specs that the builder may require. When you start getting into highly specialized guns, the 80% route may eventually become cost-effective the more you build. 

I am sure you are reading this article wondering how much this build cost. To address your curiosity, I have posted a table listing the costs of this build. Of course, there are some tools that I already own that are not on that table. If you would like to know all of the tools that I used, please visit the article Tools used in the P320 80% Videos that lists the tools used and what step I used them in. 

The Solution

If you do an internet search on P320 80% you will find multiple sites that sell the MUP-1 (80% P320 FCU) frame and jig kit from JSD Supply. At the time this article was written, this is the only Sig P320 aftermarket frame available. I decided to purchase the frame and jig kit directly from JSD Supply because of a memorial day coupon for free shipping. The price on their site is the same as any other site but with free shipping, I saved a few dollars. Actually, I saved about $27 since the code was for free shipping no matter what service I chose… so, of course, I chose next day. My order from JSD Supply included the P320 80% Frame and Jig. I did not order the lower parts kit as it was sold out so I had to do some searching for those parts. But we will talk about that later in the article.

Jig Issue #1

There are 4 screws that hold the jig together. I removed the 1st screw with no issue but when it P320 Jig Screws Stripedcame to the 2nd, I ran into issue #1. What I can surmise is that someone torqued the screw so tight that I stripped the head when trying to remove it. I decided to continue and bypass this screw and move forward. I became really worried when I got to the 3rd screw and the same thing happened. Moving on I easily removed the 4th screw just as the 1st. 

To be honest, I was getting a little frustrated and was having second thoughts about doing this video series. I wrote an email into JSD Supply at 7:43 PM letting them know about the issue.  Now that I had the release of frustration I decided to move forward with another video series and revisit the P320 80% later. Little did I know I was going to end up doing both series at the same time. 

To my surprise, I got an answer from JSD Supply 3 minutes later. And after a quick dialog, they offered to replace the jig when the next batch was ready in 4 days. I told them I was going to reattempt removing the screws in the morning using an EZ out and may not have to take them up on the offer. 

The next day, I tried once again and broke 2 magnetic tip #2 Phillips bits, but finally, the screws were out without having to use the EZ out. Fortunately, the tips I broke have a lifetime warranty and a quick phone call to Brownells had them replaced.

Jig Issue #2

P320 80% jig drill bits in holesNow with that disaster avoided, I can move forward, right? That’s when I ran into another issue.

I started laying out the drill bits that came with the jig and making sure I understood where everything fit when drilling. I found that the 9mm and #40 drill bits are too large for the holes in the Jig. At 9:38 am off went another email to JSD Supply telling them I got the screws taken care of and asked “It looks like the 9mm and #40 drill bits are too large for the holes in the jig. All of the other bits fit their respective hole. Is this normal?” 

P320 80% Large drill bit hole

P320 80% small drill bit hole








Once again, the stress release of sending an email was allowing me to move on with my day. 2 minutes later I heard the ding of an email message on my computer. I checked my email to find a reply from JSD Supply stating “Yep sometimes the variance in machining and drill bits overlap.  You will be good.” I am beginning to think that they are on the other side of the computer just waiting for someone to email them no matter what time of the day it is.

With my confidence restored that no matter what happens on this project, JSD Supply will be there to help me.

Now it was time to find my source for the lower parts kit.

Lower Parts Kit

After you have your Jig and 80% frame you need the “guts” or what is commonly known as the lower parts kit. JSD Supply was out of stock when I ordered the frame and jig (so was Ghost Guns and every other place I ran across in my searches). This left me with a dilemma, I could either put the entire project on hold or split the project into 2 videos and continue moving forward in steps.

Desperate to continue, I tried to source the parts individually on Brownells and Numrich and quickly found I couldn’t get all the parts I needed yet alone find all of the upgraded parts for the drop fire issue.

In my searches, I saw a posting on Calguns about building 80% 320s and people complaining about getting parts. In those postings, a person was saying that they can source parts. I checked out their website (ABprototype.com) and sent them a message at 3:59 pm and to my surprise, I had a response at 4:19 pm. 

Lower Parts Kit Finally Found

After a few emails with Alex over at AB Prototype, he able to get me a refurbished lower parts kit with the upgraded parts. In our email chains, I learned a little about why some of these kits may be so expensive other than the P320 is a relatively new gun on the market and parts are still scarce. He is pulling the Lower Parts out of complete pistols to make a Sig p320 80% kit and left the rest of the gun to try to sell.

If you are having problems finding your Sig p320 80% kit, drop by ABprototype.com and see what Alex can do for you. When you are ready to install them, visit our P320 80% Parts Kit Install Video to learn how to install them into the FCU after you have finished your frame. 

Now that lower parts are more readily available, JSD Supply also keeps them in stock. You can purchase them here along with a complete build kit.

Understanding Drill Speeds

P320 80% Kit Drill BitsTo Build the 80% P320 frame you will need to use 7 different drill bits (my kit came with 7 HSS drill bits) and a drill press.

One thing to understand about Drill Press’, you need to operate them at different speeds for the different drill sizes. So you will need to do a little bit of math to select the correct speed for each drill bit. I used this formula to figure out what speed to use on my press, (SFM X 4) / Drill Bit Size in Inches

I did the math already for you and the following table will help you with selecting the speed of the drill. Thanks to switchpod on the Full30 Forum for helping me understand how to come up with the correct speeds. These are calculated for HSS drill bits (the cheapest drill bits available). If you have a different drill bit type (Cobalt, TIN, etc.) you will probably need a different formula. 

Setting up the drill press

Now that we know the speed of our bits we can set up our drill press for the first hole. Chances are that your drill press will not have the speeds listed. You will need to run your drill press at the closest speed listed and when in doubt, go with a lower RPM. For instance, the #40 bit should run at 2,041 RPM and my press has the closest speeds of 1,850 RPM and 2,260 RPM. This means I will use 1,850 RPM and drill slower (apply less pressure and taking longer to drill).  

The important thing to remember is the faster a bit turns, the hotter it gets. Also, heat dulls bits quickly. Generally, it’s a good idea to drill through metal using a slower speed. Hard metals like steel, as well as, larger drill bits require slower speeds than soft material like wood and smaller bits.

To allow my drill bits to last even longer, I used cutting oil when drilling since it reduces heat buildup and friction. Also, I like to lift my drill bit to clear out the chips, cool the bit and apply more cutting oil. When drilling this frame you are not really drilling deep enough to do this however, I tend to follow this practice anyway.

What I would do differently

After test firing, I found a couple of things that I would do differently in the beginning. 

First, I noticed a good amount of wear on the frame where the trigger bar was rubbing. I would polish the frame a bit more in this area. 

Second, I would round off the edges of the frame. I found that the frame rubbed on the slide. 

Other than these two items, there are no other things that I would do differently. The build went well and was fairly easy to do. Not to mention, the gun has proven to be reliable so far. 

Other P320 80% Content

We have the most complete coverage of the MUP-1 (80% P320 FCU) P320 80% build process on the Internet. Make sure you visit our P320 80% Frame Completion, P320 80% Parts Kit Install and P320 80% Range Testing videos. Also, visit our Sig P320 80% Frame Rail Calculator to help you trim the frame rails and our P320 80% Build Adventure article to read about the problems that we ran into during this build. 

Our videos and articles will help you complete this build and show you some of the areas that can be problematic. The P320 80% build is not hard to complete and we consider it an intermediate skill project. 


If you would like to know more about the tools that we used on the Sig P320 videos visit https://trb.fyi/tools-used-in-the-p320-disassembly-and-reassembly-videos and visit our troubleshooting guides at https://trb.fyi/troubleshooting/ while you are at our page.

Other P320 Videos that are not 80% builds

If you are not interested in building the MUP-1 (80% P320 FCU) P320 80% pistol but would like to know more about this firearm, visit our P320 Field Strip, Slide Disassembly, Slide Reassembly, FCU Disassembly/reassembly, as well as our Fire Control Unit Removal and Grip Module videos.

In Closing

This was a fun build to do and I really enjoyed doing it. Building an 80% P320 is pretty easy to do and can be completed in a weekend. If you are considering to go down the road of doing an 80% firearm, I would say that this is a good one to do to build your confidence. And since this is the only Sig P320 aftermarket frame, why not buy it directly from JSD?

Some people have asked me if this build can be done with a hand drill instead of a drill press. I would say you probably could but would not recommend it. If JSD Supply built the jig with some hardened steel inserts I would say you might get away with a hand drill. 

As you can see in the following table, this build did cost a significant amount of money and your support allows us to keep producing content. 

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If you like the work that we do here at The Rogue Banshee, please consider supporting us for free by clicking on our affiliate links before purchasing gear, parts and tools or donating to us on Patreon. Your support allows us to continue to produce the quality content that you expect from us. 


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