Howdo you unlock the infinite ammo skull for story mode I will need it..

You will need to get the bandanna skull which is located on the mission the silent cartographer. This skull is located at the first facility you enter, you will need to grenade jump for it. As you enter the area instead of activating the device in the centre of the room, move to the right and you will see an open area, as you walk out look to your right and on the platform at the very end will be the skull, this is the tricky bit. Throw a grenade on the floor on which you're standing a few feet in from for you, I suggest a plasma grenade for this, time the explosion and run forward, jump just before the explosion and you'll catapult up onto the platform, don't give up if you fail, this took me 12 attempts but got there In the end. If you can't find it then let me know.

Dan

You can even d this whit a rocket launcher jump and in the air fire to the ground and you lose almost full health but then you get at the bandanna skull after you walk on something that looks like a bridge

The first guest did a pretty good job of describing where to the Bandana skull at, but just in case you didn't follow, here's a video tutorial. I didn't make it, it's the one I used to find the skull though.