Several of us have used the polymer mags that used to be sold for the SIG-556, after we discovered that they worked just fine for the 450b right out of the box, with no mods or adjustments. They hold 11-12 rounds. Unfortunately, SIG no longer sells them, but I've found out that you can get the exact same mag (and it works) from IMI Defense! If you want to try a polymer mag to help with feed reliability and higher capacity, here's a link to one source (there are others)...the good news is that the IMI mags are less than half the price of the old SIG ones:
https://www.cheaperthandirt.com/product ... rom=Search
http://www.acesammoandmags.com/IMI-Defe ... 355630.htm