Heres how it's done in the defense map:
<projectile directions="4" collision="0.85" damage="25" speed="1.1" hit-sound="sound/misc.xml:trap_fireball_hit" behavior="neutral">
<behavior>
<dictionary>
<entry name="range"><float>50</float></entry>
<entry name="category-override"><string>Player</string></entry>
</dictionary>
</behavior>
.....
</projectile>
The "category-override" thing means the projectiles count as being owned by players no matter who shoots them.