I have a P320 .40 with an X-carry frame and a compact slide. I have never had an issue with it, 4 years and multiple thousands of rounds. Shooting it today the slide started locking back midway through the magazine, with rounds still in the mag.

My thought is my thumb or hand was hitting the slide catch during recoil and bumping it enough to catch the slide, but it has never happened before, and it kept happening when I made a concentrated effort to keep clear of the slide catch. It happened with two different mags and a dozen types of ammo.

Anyone dealt with this issue before? Is it mechanical or do I just need to keep my meathook hands further from the slide catch?

Is the slide lock lever still under spring tension, or does it just flop around, and thus be able to sometimes engage due to recoil?

Remove the slide. Hold the grip and FCU upside down, and see if the slide lock lever stays in place, or if it flops around.

There's a small spring that provides tension on the slide lock lever, to keep it from inadvertently engaging when the gun is fired.

That spring is wrapped around a small black buffer that is only held in place with a loose friction fit on the underside of the front of the slide lock lever. It's possible for the spring and buffer to fall out when the FCU is removed.

(I know this from personal experience, because I ran into this with one of the used P320s I bought. The slide would randomly lock back during firing. I discovered that this spring and buffer had fallen out at some point prior to me buying it.)

Still under tension, I can see the spring in place. The fcu has been out, but not recently. It’s still the old style slide stop.

With that much shooting, maybe the spring is weakened and still allows it to move under recoil.

I had something similar happen with an AR15 and it was the bolt catch spring getting kinked up and letting it flop around and lock with rounds in the mag.

I would guess if it did it with two different magazines I would first rule out user error and let someone else shoot it and see if it still locks back. If that is the case I would look into replacing the slide lock spring.

Always start with the easiest/cheapest fix.

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According to Sig's maintenance schedule, the Slide Lock Lever, Slide Lock Lever Spring, and Slide Lock Lever Post on the P320 should be replaced at 10,000 rounds fired.

The full maintenance schedule for the P320 and Classic Sig Pistols is the first sticky post in the Sig Armorer section of the forum:
https://sigforum.com/eve/forums/a/frm/f/150601935

Before you go to doing much , shoot the gun strong and weak handed.??point your thumb straight up. 99 percent of the time it’s thumbs or something the support hand is doing.