Sentry Guns

"When your base needs some heavy defence, the Engineer comes to the rescue, building automatic sentry guns that track enemies and kill them."

SGs are great, once they lock on to someone they wont let go, killing even the toughest classes in seconds. Although very vulnerable to grenades a good engineer can keep their gun maintained and ammo happy.

Although building and upgrading an SG is easy there are some advanced tricks shown here you can use to surprise your enemies.

Floating SGs
Crouching SGs
Infinate Dispensers

Floating SGs

Although it takes a bit of time to set up this allows your sentry gun to be slightly higher and firing at a larger range.

Start by building a dispenser next to a platform the same height as the dispenser (use cz2 boxes in your base for example). If you have a friend get them to build a dispenser/sg so you can stand on that.
If the platform is slightly higher (if your on an sg) try crouching.

Now try to build a sentry gun upon your dispenser, if successful you will be able to finish building and upgrade it.

Blow the dispenser and watch as the tripod base fall to the ground leaving the main gun in the air.

With practice you can build upon stacked dispensers, but if you try to blow more than one dispenser high your sg will be destroyed.

Crouching SGs

So far I have only managed to get this to work on the lift up in the attic on Well.

Build an SG under the lift as normal.

Watch as the lift comes down and pushes the main gun into the tripod.

Make sure it is well maintained as the lift will take one point of damage off the SG every time it comes down

This is a great way of catching enemies coming up it will be almost impossible to avoid being shot unless you know about it and use grenades to destroy it.

Infinate Dispensers

This trick is essentially useless because it involves help from the opposing team. What you need is a map that has an auto-minigun usually the ones in the respawn areas. You also need to be an engineer.

Build a dispenser so that it is in clear site of the auto-minigun. Get a friend on the opposite team to duck behind the dispenser. The auto-minigun will shoot the dispenser down to 0 health. If your friend stands too close to the dispenser it blows up so it may take some practice. Now that the dispenser has -0 health the engineer can go around and build as many dispensers as he wants. Makes for a nice large explosion.
[Thanks Darknight]