Unfortunately, you can't just 'add collision' to a projectile. We would've been doing that ages ago instead of having a piece of hair flying through the air that looks like a projectile.
The collision of a projectile is the center of the grid/shape origin of the projectileIts as easy as importing the spear model, and moving the tip to the center of the grid and setting that as the origin point
That would look really odd when throwing it though. The spear would go from being held at the middle, then once you release, it'd be thrown by the tip.
It's all about animation illusion. That's why you make the projectile size 0 for the first couple frames and then at like frame 4 or so make it normal sized - giving off the illusion of being thrown from the middle/further down the handle and not from the tip
