WESHOOT2:
I just pulled my USP40 Compact out and looked at it to better understand what you were saying. It is not obvious why this would be the case. Could you please explain further. I thought that I had a recollection of Larry at CCF making a 40 to 9 conversion ... but I could be wrong. Dropping 9mm rounds into my .40 mag showed me one reason to go this route. If they feed reliable you get an easy 13 rounds in the 10 round mags ;>) and it felt like it was made for the 9mm.