P320 SXG CAL-X KIT

[Henryboy]

When you say fiddling around are not all P320 FCU interchangeable minus barrel, magazine,frame.?

The takedown safety lever is different for the 10mm-45ACP FCU. If you use the 9mm FCU in an X-ten or 45ACP P320 the magazine will not fit. If you use a 10mm-45ACP FCU in a 9mm P320, it will "work", but it will override a safety mechanism of being able to load the firearm with the takedown lever not properly installed. Sig states that these parts are not interchangeable. That being said, I don't know what other FCU parts would be different other than that takedown safety lever. You can see the physical differences at 1minute mark in the video I included here, but I don't recommend doing what the guy is demonstrating as it creates an "unsafe" firearm. (putting an Xten/45ACP FCU in a 9mm gun).

The extra real estate taken by the tab prevents you from inserting a magazine while the takedown lever is released in the 9mm version. It also consequently blocks the longer (front to back) 10mm magazine from inserting all the way up even if it is used in an Xten grip. Hope this answers your question, but I think we've highjacked this initial post... lol