A mini-gun on a tri-pod stand that works with bot events. The sentry is able to be moved by any player, team players if it is on a team vehicle spawn. Its bullets would do small damage but will be fired at a high rate. One bullet would take away 10 HP. It's size would be half the height of a blockhead and two times wider than a blockhead.
The MW2 mini-gun is the only thing I could think of that looked like my idea. :/
(Click to enlarge.)