I use sensitivity 4. Anything more and the use of snipers becomes problematic. It's a bit small for shotties, but all in all a best all around sensitivity. If you have the nerves for it the best option would be snipers 4 and shotties 6 (max), but i simply don't like to change stats every two games. People with 10 are just crazy. I know only one really, really good player, who uses 9. If i have to give advice - use the highest sensitivity setting with which you are still accurate and can land those headshots. If you start missing a lot of shots, lower immediately.