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We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, New Jersey, New York, Pennsylvania, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.

How to Build an 80% P320 Pistol

Posted by 80 Lower Jig, Co. on Jul 2nd 2020

How to Build an 80% P320 Pistol

You want a Sig Sauer P320. Good choice. But do you spent a ton of cash (and dealer fees, transfer fees, higher taxes, and the cost for a background check that might get delayed) on a "retail-bought" pistol? No. You want to build one yourself. Also a good choice. Today we're breaking down how to build an  80% P320 handgun from a compatible pistol kit. Before we show you the steps you need to take, let's answer some questions you probably have.


P320 Build FAQ

Is building a P320 with an 80% frame legal?

Federally, yes. An individual who can otherwise legally own a firearm may build a firearm at home for personal use. The firearm in question cannot be made with the intent of selling it, or that'd make you a manufacturer. The ATF says you cannot build a non-sporting semiautomatic rifle or shotgun if it has 10 or more imported parts, but we don't need to worry -- we're building a handgun and everything in the build kit is US-made. (ATF Source).

What about state laws?

Some states have banned gun owners from making homemade firearms using kits like the Sig P320 build kit, and others (like California) require you to submit to a background check and request a serial number for your firearm. It's also illegal to build a semiauto handgun in California if you don't pay thousands of dollars to have it "certified for sale." Sorry, Californians. We don't like it either.

Currently, these P320 kits cannot be shipped or built to: New York, Connecticut, New Jersey, Washington state, Washington D.C., and Rhode Island.  We cover most relevant laws in more detail here.

Do I need a license or FFL to do this?

Federally, no. Only manufacturers require licensing as Federal Firearm Licensees (FFLs) to manufacture firearms that are made for sale to others. You're building your P320 for personal use, so you don't need a license. Click that "ATF Source" link above and it confirms this, too.

What part of the P320 is considered a firearm?

The ATF will most often classify just one component of a firearm as the actual "firearm" under legal definitions. Everything, like the barrel, trigger parts, slide, and all other parts (in the case of the P320, at least) are not considered firearms. Sig Sauer calls the firearm component of the P320 a "fire control unit" or chassis. It's called the MUP-1 P320-Compatible Insert, courtesy of our partners over at JSD Supply. This is the part in your 80% pistol kit that you'll be fabricating into a firearm yourself.

Is this kit compatible with factory Sig parts?

Yes. The 80% kits we offer up for this build use OEM parts and factory-spec components made from the same materials and coatings as a retail-purchased Sig P320. That means with your finished pistol, you can modify your chassis to accept any of the P320's available calibers and four frame and slide sizes. 

What are the P320's available configurations?


The Sig Sauer P320 can be chambered in 9mm Parabellum, .357 SIG, .40 S&W, and most recently, .45 ACP. The slide and grip module (frame) sizes allow the pistol to be configured in a Full-Size, Carry, Compact, and Subcompact profile.

We provide kits which are available in a Carry or Full-Size configuration, both chambered in 9mm Parabellum.

What tools do I need to build my pistol's kit?

Fabricating the 80% fire control unit requires some basic hand tools you may already own or have lying around:

  • Dremel
  • Drill press
  • Rubber mallet
  • Cut-off wheel for Dremel
  • Table-top vise or drill press vise
  • Basic machining oil or lubricant

You'll be drilling small holes into some relatively thin steel, so you don't need a large or bulky drill press. Some builders choose to simply by a vertical drill stand for their Dremel to get the job done.

Let's dive into the instructions.


How to Complete the 80% Fire Control Unit

Our friends at JSD Supply put together one helluva great visual tutorial that shows you exactly what steps you need to take to cut and drill your 320's MUP-1 fire control unit. Simply secure your FCU in the jig as shown, tighten it down in your vise, and begin with the video. First, you'll use the jig's top plate and rubber mallet to bend the slide rail tabs. Then it's a matter of setting up your Dremel, cut-off wheel, and drill platform.

To aid in the fabrication, you can download the MUP-1 jig instructions here

Take some time to read through before starting the video and fabrication steps. The written instructions provide some important details and best practices you should follow to ensure your finished chassis is reliable and ready for install.

MUP-1 Jig Instruction Video


Assembling Your P320's Completed FCU

With fabrication completed, your finished FCU is ready for your pistol parts kit's components to be installed. First, you need to assemble the trigger, trigger bar, striker, sear, disconnector, slide release, and safety mechanisms. This requires the various springs and pins included with the kit, so it helps to have a small gunsmithing hammer and roll pin punch set. 

80% P320 FCU Parts Kit: What's Included

  • Trigger stop pin
  • Slide catch lever pin
  • Safety lever pin
  • Sear pivot pin
  • Split coil pin
  • Slide catch lever spring post
  • Take down safety lever
  • Safety lever
  • Slide catch
  • Sear
  • O-ring
  • Take down lever
  • Slide catch assembly
  • Spring, slide catch lever
  • Spring, sear (2)
  • Trigger bar spring
  • Spring, take-down lever
  • Trigger
  • Trigger bar
  • Sear housing
  • Manual disconnector

P320 "Exoskeleton" Parts

With the FCU assembled into a working unit, all that's left is to install the grip module (frame), barrel, recoil spring with operating rod, and slide. These five components are what you'll be swapping out to modify your P320's caliber and size configuration.

JSD's final assembly video details all the steps.

MUP-1 Pistol Assembly Video


Your P320 is Complete!

With your parts kit installed and your exterior components fitted to the chassis, your Sig Sauer P320 build is ready for the range. You can quickly and easily modify your configuration or caliber by swapping out the exoskeleton components. Disassembly is simply the reverse of the install, and it requires no tools. 

Configuration Specs

Full-Size

  • Length: 8.0"
  • Barrel length: 4.7”
  • Width: 1.4”
  • Height: 5.5”
  • Weight: 833g (29.4 Oz.)

Full-Size Capacity

  • 9mm: 17 rounds (Tacops Full affords 21 rounds)
  • .357 SIG: 14 rounds
  • .40 S&W: 14 rounds
  • .45 ACP: 10 rounds

Carry

  • Length: 7.2”
  • Barrel length: 3.9”
  • Width: 1.4”
  • Height: 5.5”
  • Weight: 737g (26.0 Oz.)

Carry Capacity

  • 9mm: 17 rounds (Tacops Carry affords 21 rounds)
  • .357 SIG: 14 rounds
  • .40 S&W: 14 rounds
  • .45 ACP: 10 rounds

Compact

  • Length: 7.2”
  • Barrel length: 3.9”
  • Width: 1.4”
  • Height: 5.2”
  • Weight: 737g (26.0 Oz.)

Compact Capacity

  • 9mm: 15 rounds
  • .357 SIG: 13 rounds
  • .40 S&W: 13 rounds
  • .45 ACP: 9 rounds

Subcompact

  • Length: 6.7”
  • Barrel length: 3.6”
  • Width: 1.3”
  • Height: 4.7”
  • Weight: 708g (25.0 Oz.)

Subcompact Capacity

  • 9mm: 12 rounds
  • .40 S&W: 10 rounds
  • .45 ACP: 6 rounds

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.