From past experiences, (multiple accounts of trying out different weapons). I have found that when you level up to sgt with a s/d over 2, you will be kicked out of 1-10/ 1-5. You cannot be let back in once you are out. Oddly enough, as i progressed throughout the staff sgt tiers, i had a s/d of 2.3 but still played in 1-10 server. It is possible that there is a limit that i have not reached