Unless I read it wrong (and I might have), Top Gun now has an XGrip adapter for a sub compact to take full size mags. I had to get a sc module for my compact to get the small size since Sig seems to be putting a low priority on supplying parts for the 320 anymore (at least Grip Modules and magazines). I plan to carry the 320 with the 12 round mag and have the 21 mag as an extra if the adapter works out.