I have had a time trying to get extra 21 round magazines for my P320 XFive 9MM pistol.
I have ordered from two different vendors and received the wrong mags,each time, even after the salesman on the phone swore they knew exactly what I needed. The mags they send me are the 21 round mags made for the P320 Tacops 9MM and will not fit the XFIVE, Because the base plate is made different.(please don't tell me to buy the wrong mags and then go buy the xfive baseplates)
I finally called Sig and they sent me the right mags and understood that there are two different style 21 round P320 magazines made and the base plates seem to be all that's different, However I wanted the right mags and did not want to pay for extra base plates after buying the wrong mags.
Here is the photo of the correct mags for the XFive so that others will be able to get what they need without the headache I have had.

It appears, but I cannot confirm that the new style 21 round mag will work with both the P320 Tacops and the P320 XFive.













The below 21 round mag I have pictured will not work with the P320 XFive pistol.
The base plate is wrong. It is made for the P320 Tacops pistol.

This message has been edited. Last edited by: Rockyriver,

 
One other option, may or may not be available to you. I have a regular full-sized P320, with a handful of 17 round mags. I replaced the base plates on all of them with Springer aluminum 1/4" plates, which left me with a pile of stock 17 round base plates. Then I bought a 21 round mag, which would not work with my Springer magwell. Rather than modify the base plate on the 21 rounder, I just put a factory 17 round base plate on it, which does not have all that extra outside plastic rising up from the bottom. Perhaps not as attractive, and won't work if you don't have a source of 17 round base plates laying around. But, another choice.
 

Good to know! (an X5 has been on the want list at some point)

My older 21rd mags were ordered as P320 21rd mags from Sig direct at some point last year. They have the TacOps style (straight edge) baseplate.

I just ordered a couple from Brownells last week as P320 21rd mags and they came with the newer (beveled) style baseplate. I wonder if they'll just make those from here on - one size fits all!

Thanks for explaining the difference for those of us that don't have both.