Same as a MAC but since the one on harvest was called a driver. I made mine around that idea.
Ive had this idea for a very long time but only recently have a learned enough forge techniques to put it into working practice.

Mass Driver is set on the edge of a cliff and fires back along the map.

For the Tungsten shell im using a crate, as it was simple to create a barrel for it to fire along.

And this is where the crate generally lands. I soo wished i could have gotten it further. I had tried experimenting with fusion coils to try and give it a boost but this seemed to make the crate fire out of control and mostly land closer.
Also more fusion coils makes the cannon more easy to trigger manually.

This picture looks cool. I made the cannon with look in mind. You can see fusion coils in small compartments lined across the top. This is to make a controled explosion along the top.

With look in mind i placed it overhanging a cliff. I merged two ramps into the cliff to make it look supported and used watchtower bases to make a cool way up to the control room. The is a small garage and a landing pad also.

You can be fired out of it. Either by the crate or by walking through the man cannons. you land just short of where the crate lands. If you want to be fired out go through the opening and stand next to the crate.

(Parts cut out to show inside)
To trigger the cannon go to the control room and crouch in the pit. Its right in the centre of the room, cant miss it. You pick up the custom power up which releases the fusion coil. This starts explosion.

The crate is held back by power cores. when explosion reaches them they blow up and lets the crate free. Simple really but soo tricky to get the holes in the shaft and everything working smoothly.
The crate is pushed by my version of a railgun or Mac. I discovered a while back that if you use two man cannons to push an object through like a bar of soap they go much faster.
Three sets of dual man cannons power the Driver.

The control room lets you look up the barrel of the gun and provides accsess to the opening. Firing mechanism is the hole in the floor in the middle.

I used wall corners and ramps to creat ridges or shock absorbers for the cannon. There are alot of shields used. this is to stop people from accidentaly setting it off.

The entrance to the gun is bellow the shaft and if you ask me looks sweet.
A short tunnel leading to a garage. I merged the ramp perfectly with the cliff.

Download Mass Driver http://www.bungie.net/forums/posts.aspx?h3fileid=44755827