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80% P320 Compatible Insert

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The P320 by Sig Sauer® was a great addition to our industry. It introduced the idea that you could have one pistol, but that you could modify it to be adaptable to any carry situation. They did it by defining the insert or the "guts" of the P320 as the legal "firearm," so that you could modify everything around it, thereby changing its carry size and profile. If you're looking for a new pistol with that same end-all/be-all feel to it, that same one-and-done idea, but you want to build it yourself and without that new pistol price, then you too need to start with the piece the government considers the "firearm."

We're excited to bring you that piece. The P320-compatible MUP-1 comes to you in 80% form. When you've completed work on it, it too will legally be defined as the "firearm" itself, which will then make it as modular as the expensive one you'd buy at your LGS. When outfitted with P320 compatible parts, the insert can be used to make a 9mm, 40 S&W, 357 SIG, and 45acp (with slight modifications).


  • Made from tough 316 stainless steel to stand up to years of abuse
  • can be used to make a 9mm, 40 S&W, 357 SIG, and 45acp (with slight modifications).

Steps Remaining

To finish this insert and make it ready for use, the following steps must be completed:

  • Drill all holes
  • Bend side rails
  • Trim rails to size

To see a great video on how to install a lower parts kit onto your P320-compatible insert, check this out.

DISCLAIMER: “Sig Sauer” is a federally registered trademark of Sig Sauer, Inc. and is one of many trademarks owned by Sig Sauer, Inc. Neither 80% Lowers, a/k/a 80-lower.com 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 www.sigsauer.com.

WARNING: Handling firearms parts and accessories can expose you to lead and other petroleum products known to the State of California to cause cancer and reproductive harm. For more information go to: www.P65Warnings.ca.gov

Shipping Restrictions

Shipping of various parts and/or accessories are prohibited to states and locales (ie New Jersey, California, New York, etc…) with restrictions against certain parts and/or accessories or "assault weapons" (i.e. states with high-capacity magazine restrictions, manufacturing of semi-automatic weapons, etc..). It is your responsibility to be aware of your local firearms laws before purchasing from our website or machining a firearm. We try our best to keep our shipping page current, but laws change daily.

80-Lower.com only ships within the United States. Federal legislation dictates we cannot ship our products internationally.

SIG SAUER® Compatible

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.