It is here: (Thank you Dragoncoals for nice youtube BBC code! ;D) [youtube]http://www.youtube.com/watch?v=3ldiX0z2nxY[/youtube]
But in real life it is a magnetic accelerator cannon that shoots metal {forgot teh type} rods at 4000-5000 feet per second.
Actually in real life we don't have MACs that powerful yet. Here is a quote from halopedia: The standard ship-mounted MAC fires a 600-ton ferric-tungsten projectile with a depleted uranium core at 30,000 km/s (this is circa 2525 - MAC technology might have improved since). The large amount of energy needed to fire the weapon is particularly onerous on a warship, and the extended recharge time is a significant factor in combat against Covenant warships as multiple MAC rounds are required to penetrate Covenant shields. The standard MAC is sufficient to destroy any human vessel or severely damage an unshielded Covenant vessel. End quote
Actually, they had a thingy about it on future weapons. we are still trying to make it so ships have teh powersupply to use it. so it is still under construction. [youtube]http://youtube.com/watch?v=y54aLcC3G74[/youtube] its a long vid, but hey.
I made a railgun in foundry using fences for the cage. I then placed a wall of teleporters in the flight path of the railgun. The receiver node returns any (un)willing victim to the start of the railgun, with a well-placed gravlift preventing their escape. Above the wall of teleporters, I spelt the word 'LOL' with fusion coils and a soccer ball, so that those stuck inside the infinite loop have some reading material for every journey. I then cleverly disguised the whole thing by hiding it from view of the spawn points using double boxes, fencing off the immediate area, placing some hammers, mongeese and soccer balls on the other side of the fence next to a receiver node*, and placing a teleporter in front of the fence so that the unsuspecting will automatically assume the two teleporter nodes they can see are linked. *The receiver node is actually part of a three-way teleporter link that allows those in the know to access the hammers, and to then proceed to the flight path of the victims trapped in the railgun trap. Mercy comes only with the fatal blow of the hammer. If anyone would like to see this map variant, I'd be quite willing to put it back in my fileshare.
Here's one we made for fun.... I later made it longer and replaced the missing side fence someone had deleted in this pic....it's fun to pile stuff up at the far end! lol
I actually have a base made with two rail guns built in, I had it made in anticipation of a foundry-based "fort contest" over at High Impact Halo, which never really happened, apparently. I'll get it onto my file share as soon as possible, and provide a link (look for it probably later tonight)