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Sig Sauer P320 80% Build Guide

Posted by 80-lower on Nov 12th 2019

Sig Sauer P320 80% Build Guide

Sig Sauer® introduced the P320 modular handgun in 2014 and it quickly became one of their most popular pistols, ever. The U.S. Military adopted this new platform (designated the XM17 and XM18) and it now sees service globally. Naturally, we felt the P320’s modular, multi-caliber design made it the perfect candidate for getting the at-home-gunsmithing treatment. So, we introduced some Sig 80% build kits for the P320 platform. Here’s Build Guide to get you started!

What is the Sig Sauer P320?

The Sig P320 is many handguns in one. Using interchangeable parts – barrels, slides, grip modules (frames) and recoil assemblies – you can transform it into any configuration without having to do any paperwork. There are a ton of configurations available, by the way.

Available Pistol Configurations

Sig wanted to make the P320 available for nearly every shooter’s needs, so they introduced four calibers: 9MM, .357 SIG, .40 S&W, and .45 ACP. Sig also introduced modular kits for full-size, carry, compact, and subcompact builds:

P320 Full-Size

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

P320 Carry

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

P320 Compact

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

P320 Subcompact

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

Unique Features & Quick Review

The P320 has received some pretty rave reviews, averaging 4.5- to 5-star ratings across most seller’s platforms. The P320’s biggest features – obviously, aside from its modularity – are a short-reset tactile trigger; striker-fired operating system; classic SigLite Night Sights or contrast sights; integrated M1913 accessory rail (subcompact excluded); and not one but five safeties:

A striker safety to prevent accidental discharges; a disconnect safety to prevent out-of-battery firing; a 3 point takedown safety, and a magazine disconnect safety.

Converting your P320 from one configuration or caliber to another takes just a few minutes. You can swap out a full-size configuration chambered in .45 ACP for a subcompact chambered in 9mm – essentially an entirely new gun – and legally, it’s still considered the same firearm. That’s because the “core” of the P320, the firearm itself (by legal definitions), is a universal chassis that works with all the P320’s available parts. This genius little device is called the Fire Control Unit:

Sig 80% Fire Control Unit

Since the FCU, insert, lower, chassis – whatever you want to call it – is the only part considered the firearm in the P320, this is the component we re-engineered with our partners and fellow gunsmiths. We now have a quality unit available for the P320 called the MUP-1, or “Modular Universal Pistol-1” 80% insert:

It’s made from 316 stainless steel and boasts the same dimensions and specs as a factory chassis inside a store-bought gun. The MUP1 is compatible with the P320’s “drop-safe” upgraded parts and all calibers (.45 ACP requires slight modification).

Transforming the MUP1 into a legal-by-definition firearm means cutting and machining it much the same way you would the typical 80% lower for the AR-15. To do that, you’ll need the 80% P320 jig:

The “Sig Jig” includes everything needed to drill the right holes, open up the trigger slot, and bend and trim the slide rails. Check out the quick video to see how the jig works.

MUP1 Jig Instruction Video

This completes the fabrication required to turn the 80% insert into a functional firearm, ready to accept a lower parts kit and trigger, and build kit. At this point, you own a legal-by-definition firearm that isn’t functional – it’s equivalent to owning an AR-15’s stripped receiver.

Assembling the Completed FCU

Once your FCU/insert is machined, you can make it ready to fire by installing the P320’s lower parts kit. Since the completed insert is universal, so it’s the parts kit. The kit includes everything needed to drop the insert into any configuration:

  • 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
  • Oring
  • Take down lever
  • Slide catch assembly
  • Spring, slide catch lever
  • Spring, sear (2)
  • Trigger bar spring
  • Spring, takedown lever
  • Trigger
  • Trigger bar
  • Sear housing
  • Manual disconnector

Completing Your P320 Build (Choosing a Configuration)

At this point, you technically own a functional Sig Sauer P320! It just needs the right suit of armor to complete the look and send live rounds downrange. To do that, you’ll need to buy a P320 Build Kit which means choosing your pistol’s configuration – full-size, carry, compact, or subcompact. All the available build kits include the grip module (frame), slide, barrel, operating rod and recoil spring, magazine and preinstalled sights required to complete the build.

The Legal Stuff

Building a gun from scratch is legal to do at home in all 50 states. However, some states – like California and New York – have severely restricted or outright banned certain gun-making kits like this, so you’ll need to double-check your local and state laws before starting this project.

At the federal level, the ATF says this is legal:

Q: “Do I need a Federal firearms license to make a firearm for my own personal use, provided it is not being made for resale?” 

A: “Firearms may be lawfully made by persons who do not hold a manufacturer’s license under the GCA provided they are not for sale or distribution and the maker is not prohibited from receiving or possessing firearms. However, a person is prohibited from assembling a non-sporting semiautomatic rifle or shotgun from 10 or more imported parts, as set forth in regulations in 27 CFR 478.39. In addition, the making of an NFA firearm requires a tax payment and advance approval by ATF. An application to make a machinegun will not be approved unless documentation is submitted showing that the firearm is being made for the official use of a Federal, State or local government agency (18 U.S.C. § 922(o),(r); 26 U.S.C. § 5822; 27 CFR §§ 478.39, 479.62, and 479.105).”

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, 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.