Based off the fact that it appears the track is two stages I think that all I will need to do is simply take the top off down to the next step and there are two channels on either side that will support the spear very lightly. You can see it in your gun, but this is the best I could get with my iphone...
You can see the little chip in the second 'level' of the barrel.
You can see them best here in this photo. I put arrows on the track. This part here lines up perfectly with the track on the handle, so this will likely be what the spear will rest upon.
Petros said that they made this gun with the idea that it could be convertible. Uncle Petros also said that if I beg long enough, I should be able to arrange replacement barrel. With that in mind, I think today Im going to make this an open track. Based off my meticulous calculations (which is nothing more then a crayola picture on a bar napkin ) I shouldn't have to do anything other then that.