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The SIG P320 is one of the greatest modular handgun platforms to hit the market. Introduced in 2014, the P320 uses an innovative "universal" firearm chassis, capable of swapping three calibers and four sizes for a plethora of applications: Choose 9mm, .357 SIG, or .40 S&W and pick between a Full-Size, Carry, Compact, or Subcompact barrel, grip, and slide. Of course, such a platform deserves the at-home 80% receiver treatment. With one of these SIG SAUER® Compatible Pistol Kits, you get the ability to configure every type of handgun you could ever want or need.

The MUP1 80% Fire Control Insert

Like the 80% lower receiver for the AR-15, the P320-compatible platform utilizes an 80% version of the Sig's factory chassis, called the MUP1 FCU insert. Completing the final manufacturing steps of the insert transforms it into a firearm by legal definition. With the MUP1 insert drilled and machined, final assembly of your complete pistol takes just a few minutes. Then you're ready to carry or hit the range. 

Read the full guide on how to fabricate and build the 80% P320 Kit.

State Laws Vary

Some states have restricted firearm build kits and 80% lowers, contrary to federal law. Other states consider 80% receivers (firearm blanks) to be firearms. Before placing an order, review our Shipping & Returns Policy to ensure your state is not restricted. We cannot fulfill orders placed in restricted states. If you have questions about the legality of building a firearm in your state, always speak with a licensed attorney first.


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.

DISCLAIMER: “Sig Sauer” is a federally registered trademark of Sig Sauer, Inc. and is one of many trademarks owned by Sig Sauer, Inc. Neither AR-15LowerReceivers, a/k/a Tactical Gear Heads, LLC nor this site are affiliated in any manner with, or otherwise endorsed by, Sig Sauer, Inc. The use of “Sig Sauer” on this page is merely to advertise the sale of Sig Sauer pistols, parts, or components. For additional genuine Sig Sauer, Inc. products and parts visit

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, Hawaii, 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.