Only single stack .40s I'm aware of are the Kahr, the AMT Backup, and 1911 clones (the SA Compact, the Kimber Ultra Carry and Compact, and the AMT Skipper). Of these, the Ultra Carry or Kahr would be my choice. Lighter and more reliable than the rest. Personally, I'd go with the Kahr. It's got a relatively light DAO trigger, has essentially no protrusions, and is overall very nice.