I believe that to make a Projectile, you just import the model and center it upon the axis, so that the axis is the very middle of the model, in all directions. You don't need a joint, you just export the .dts as such. Now, if you were making something that spun, you might add a joint that you would animate to spin but with a dart I don't think you need that.
At least, this is the protocol for making a projectile in Milkshape. I assume it would be the same in Blender, but there might be some sort of parenting or whatever that you need to do.
I would also use an LOD of 0
Nice model, by the way.