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Finish assembling your MUP-1 80% chassis with a P320 Build Kit.

The MUP-1 chassis, or fire control unit (FCU), is the P320's core firearm component. Whether you fabricated one from an 80% unit or need to replace or upgrade components on your factory FCU, these P320-compatible build kits are the ticket. Each kit includes all pins, springs, detents, and parts required to assemble the FCU and install a grip module, slide, barrel, safety, trigger, and all other components for regular functionality.

P320 Internals and Grip Modules Available Separately

Maybe you want to simply upgrade your existing P320 -- or perhaps you own a grip module, slide and barrel and only need the internals for your 80% unit. We've got you covered with partial parts kits. Looking for a quality barrel? Grab one of our nitride-treated units for MOA accuracy. Check out the P320 "lower parts kit", compatible with 9mm, .40 S&W, and .357 SIG.

State Laws Vary

Some states require serialization of your lower before completion (here's a guide on how to do that), and some states have banned 80% lower receivers, certain firearm parts, and other products we sell. Check your local and state laws, and please confirm whether we can ship to your state before ordering.


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 Gun Builders Depot, a/k/a 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.