As Dave mentions you might want align all the verts to an edge and set the pivot to that edge clear the history and then use the mirror geometry command with merge verts on (this is the default). This is in the mesh menu or you can hold the shift and the rmb and mirror geometry is in that marking menu.
One thing to remember when you are snapping verts make sure you are in world mode and that you have keep spacing unchecked.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675