I had a strange experience today shooting my .45 320. I had a magazine that refused to function in the gun. It would fail to feed the round and I would have to drop the mag to clear the gun.
Here is whats odd:
I have 10 .45 Compact magazines total and own a P250 .45 Compact. The magazine that failed today is marked "Sig Sauer P250". The other 9 magazines are just marked "Sig Sauer". All mags are marked 45. All the mags have the older style round base plate retaining pin, not the new square one.
So... My question is this: Is there a difference between a P250 .45 magazine and a P320 .45 magazine? My P250 works with all my magazines.

I don't have an answer for whether or not the mags are the same, although my understanding has always been that they were, aside from a few minor generational differences like the baseplates.

Has that mag worked properly in the gun in the past? If so, I'd conclude that it's more of a mechanical than a design issue. I'd be tempted to tear down that mag alongside one of your other P320 mags and see if there are any obvious differences. Maybe even go so far as to break out the calipers.

I actually had a weird problem with a P320 mag last night...it was a full-size 9mm mag, which has a capacity of 17 rounds, but I could only get 16 into it. The 16th round showed in the 17-round witness hole, and I could hear rounds loosely moving around inside the mag when I shook it. The mag in question is one of my range mags and gets used frequently...this was the first problem I've ever had with it. A good tear down and cleaning and it's back up and going with 17 rounds...must have just been a bit of grit in exactly the wrong place.

Sig will tell you that magazines are a disposable item, but they're also the ones who charge $50/apiece MSRP for them, so....

Check to see that it is assembled correctly and or if anything in the tube or on the follower is hanging up.

I have seen mags NIW from the factory with springs assembled improperly so it can happen. Just a thought.

Very early P250 magazines had a different style of baseplate, as do newer P320 magazines, but the magazine bodies are the same throughout the generations.

Sounds like an issue with the magazine itself. Could be any number of things. Improperly reassembled, gunked up interior, worn out spring, bent feed lips, etc.

Give it a once-over to see if it's something that you can correct. If not, mark it in an obvious way (like with a pink or yellow paint pen on the base) and use it strictly as a training magazine in your P250.

This a second generation magazine for the 320/250 grip. I cleaned the gun this afternoon and checked over the magazine. Clean inside, properly assembled and appears identical to all my other .45 magazines. I hand cycled 9 dummy rounds through the gun / magazine, and it worked fine. Since I store all my mags together and only take 4 or 5 to each range session, its possible I have never actually used that mag in my 320 before. I have marked its baseplate so it will be range only.