I was playing Killteam last night and I love the fact that the Techmarine can deploy a tarantula and mow down legions of Xenos. This was also a feature in Dawn of War. I noticed that the Techmarine cannot do this in Deathwatch. Come to think of it, I do not recall stats for the Tarantula ever being released. This is pretty strange since that is such an iconic weapon. Should Techmarines not have the ability to deploy Tarantulas?

You can use Tarantula Sentry Gun from Dark Heresy "The Lathe Worlds" book, p.63.

Where would they be stored, twin heavy weapons, the support systems, the power source and ammo. They would take up a lot of space, how would a tech marine carry it all. He can't just have it appear at the press of a button.

You wouldn't be able to carry them, but requisitioning one to be carried in a gunship or transport and set up at an evac point (especially if the transport will be left unmanned) is a nice idea. Having one magically 'teleport in' Kill-Team style seems a bit odd.

Since they can't move under their own power, the best equivalent for 'normal' use is a coterie of gun-servitors, which I think is available.

There are stats for the Tarantula in one of the newer books. I don't remember exactly where though.

This sounds awesome and I can't believe I never thought about it. I am definitely going to use this in my game.