lgp9999

I am looking to buy a Heckler and Koch USP in the near future. I have not decided on a caliber yet, but it will most likely be either 9mm or .45. Is it single or double stack? Also I have researched the USP 9 a little and I see that in other countrys it has a 15 round mag (I'd think that'd have to be double stack), but due to US laws you can only get 10 round mags here. Is there just a block in it?