That is the follow up video I was talking about.
To understand what is going on internally, here are two pictures of the mechanism with the slide removed and the relevant areas circled to show what is happening.
Pistol in the Take-Down position: Take-down lever rotated to remove the barrel and slide, shoving the Take-Down Safety bar to the rear. The Take-down safety bar is then blocking the magazine well and holding the sear down so you can remove the slide. The Slide-Stop lever is locking that bar to the rear so you can remove the take-down lever from the pistol and remove the entire FCU from the grip module if needed.
Now, Pistol in the normal firing configuration: Take-down lever is rotated to lock the barrel and slide onto the pistol. Take-down safety bar is in the forward position and no longer holding down the sear or blocking the magazine well. The slide-stop is no longer holding the take-down safety bar to the rear.
So, if you don't follow the assembly steps in the manual, you end up with a pistol that looks assembled correctly, but the slide stop lever is still holding the Take-Down Safety bar to the rear, which is holding the sear down.
If the pistol is perfectly within tolerance, this condition just results in a dead trigger... the sear is down and can't cock the striker, the magazine well is blocked so you cannot load ammunition into the pistol. And you can clear this malfunction by simply locking the slide back with the slide stop once. At which point the take-down safety bar will move forward to the normal assembled and ready to fire position.
If the accumulated part tolerances are not working in your favor, you end up with what the fellow in the video is demonstrating... a miss-assembled pistol that cocks the striker and holds it just enough when the pistol is stationary, but will release it if you bump the slide. At least his is still blocking the magazine well, so it is giving away clues that something isn't right.
Hell of a mouse-trap design
The entire Take-Down Safety mechanism doesn't exist on a P250. It is another fiddly-bit that was created just for the P320 to be able to take it apart without pulling the trigger. If you are comfortable with the take-down procedure of a Glock, then the entire mechanism could be omitted from the P320.
And, if you are not having enough fun with this already.... Not all P320's behave the same way in this condition. The example one I used for the photograph doesn't get caught in an "assembled but take-down bar stuck back" condition. In other words, it will assemble correctly even if you skip the step of locking the slide to the rear before rotating the take-down lever. There is enough clearance with this specific Wilson Combat grip module to allow the take-down safety lever to jump under the slide-stop lever. This is an example of how one out of spec part can affect the entire mechanism if the piled up tolerances are in your favor. (I doubt Wilson Combat actually intended to defeat Sig's take-down safety system, it just happens that this combination of assembled parts have that effect.)
The Sig fan boyz are in denial about the nature of the problem. Sure, the bulk of P320 pistols in existence probably work correctly in most respects and will not fail in any way that is hazardous to the end user. But there are ones that left to factory with the built-in potential to fail in a dramatic way. Without inspecting and testing every single one, there is no way for anyone to estimate how many faulty P320 pistols are out there. Sig just doesn't know... which should scare the hell out of them. I managed to find issues using a sample size of just three pistols. It just isn't a very good design.
