This only works if you are attacking the base inside the gate opposite of the beach and if the gate is still closed. What you can do is have some teammates waiting outside of the gate with a warthog. You go into the base (which is an almost sure suicide but which will hopefully let your team score) and grab the flag, then you throw it through the gate to your teammates. They should be able to get away quickly since the other team would have to run all the way around because the gate is closed.