I've settled on M-16 with Silencer (rather than Red Dot), Stopping Power and Penetration. It's amazing how many don't see you without the radar blip, and the iron sights on the M-16 really aren't bad once you get used to them. I absolutely rock on Downpour and Crossfire. Crossfire is also easily done with the auto-shotty --every encounter is close combat.. bang bang, you're dead.
Btw - I think Showdown is an absolutely terrible map. I hate it unless it's Team Deathmatch and the enemy team doesn't watch their 6 - it's really easy to sneak up on people in that map. On Domination, whoever controls the center wins the game.
