I started playing the sniper role in our local games and found .30's work the best for my situation and guns. I would like to try out heavier Bb's and will some time in the near future but anything less then .28's are useless IMO
one thing you will need to get use to is watching a BB hit your target just to have that target look around and not see where it came from... In turn not calling their hits.
Personally I have fun playing this role and a buddy and I work well doing it together.
Also I find 1 kill with the sniper rifle feels just as good as 10 with an AEG