How do you get a sentry gun on Zombie assault 2

you get 4000 dollars for the first one after picking up the gun that looks like a browning from ww2 and buy it or buy the 10000 dollar one they come in one package adws is the second one(10000 dollars) and better it is a 50 cal. a.a. gun or anti air craft gun and you can reload it for free any amount of times you want giving it 1000 rounds unless you have sentry gun engineer like me than it is a quicker reload time for it and it gets 2000 bullets.