The Maxim Defense Industries PDW Brace AR-15 is designed for use with AR15's. It replaces the buffer tube and buffer assembly on an AR15. Since the Sig MPX does not use a buffer assembly the Maxim Defense Industries PDW Brace AR-15 will not work.
The Maxim Defense Industries PDW Brace AR-15 is designed to be used only with AR15's.
The Maxim Defense Industries PDW Brace AR-15 is compatible with the JP SCS that is proprietary to Maxim Defense. This is the JP Silence Capture Spring System XS models.
I recommend using a Heavy Buffer System for that set up.
The 9mm spring for the Maxim Defense Industries PDW Brace might work for a 5.56 but it is recommended you use the 5.56 set up.
If you do install on st victor 223. 5.56. 7 inch barrel. Can you recommend correct size and or buffer, buffer spring?
The Maxim Defense Industries AR-15 SCW Buffer and Spring that I would recommend for your setup is the H2 buffer. This will allow for consistent, reliable feeding and ejection. You can find it using the following link:
https://www.opticsplanet.com/maxim-defense-industries-ar-15-scw-buffer-and-spring.html?_iv_additional-features=h2
You would need to use the pistol caliber Maxim buffer and spring with this PDW brace. You can find the Maxim pistol buffer and spring using the following link:
https://www.opticsplanet.com/maxim-defense-industries-cqb-buffer-spring-set.html?_iv_additional-features=pistol-buffer
Thank you for your recent inquiry on the Maxim CQB Buffer. We suggest using the H1 buffer for the Caliber 5.56 with a 10.5" barrel.
No this does not.