Is the P220 .45 mag interchangeable with the 10mm mag? There aren't a lot of 10mm mags out there, compared to the older and more ubiquitous .45.

I haven't tried this myself as I bought a 10mm super early and no one else had a need for the 10mm mags at the time. They were easier (possible?) to find back then.

The consensus is that they work just fine with the .45 magazines in the 10mm.

I contacted Sig, and the reply is no. I know some people claim that the 45 mags work just fine with 10mm. I just need solid confirmation.

 
I have had no problem using the .45 mags with the SIG P220-10mm, except for one corner case. That was for Underwood 180gr Gold Dots. They remain "fat" right to the end, and the .45 mags are dimpled on both sides, so it can be a rough feed. The original 10mm mags had no dimple at all, and the updated version has as slight dimple on the right side (looking down on it), that makes seating it very easy. Either version of the 10mm mags take those Underwood rounds easily, whereas the .45 mags are not so kind.
 
Other than that corner case, the .45 mags work just fine in the 10mm. Their thickness is exactly the same. The witness holes will show you only have 7 rounds loaded when you have 8, but other than that, they work fine. Now that I have a good supply of the new version 10mm mags, I use the .45 mag for my barney bullet only, and the regular 10mm mags for the match.