I found a way to cheat around waiting to get level 20 assult for the shield. In story mode level 3 terrorist have shields, pick one up as your secondary, leave and there you go. You now have a permanent shield until you change it in terrorist hunt or story mode.
This game is awesome, what is your guys favorite terrorist hunt so far, mines kill house. Go to the roof throw a nade off the building and then by the stairs. You get at least 2-4 nade kills every level to level your assault up