The AI isn't really up to it, but having projectiles be able to lay down semi-persistent damaging particles or fragments behind them as they travel would be useful for nano, radiation and/or chemical weapons.

Currently we can only lay down non-damaging exhaust trails, or have a single release of particles/fragments as a detonation.

george moromisato 18 May 2021:

In 1.9 Beta 3 you can add fragmentInterval="10" to create fragments every 10 ticks (or whatever). E.g.,:

<ItemType ...>
   <Weapon ...
      fragmentInterval="10"
      ...
      >
      <Fragment
         count="..."
         ...
         >
      </Fragment>
   </Weapon>
</ItemType>

In 1.9 Beta 4 I made some tweaks to improve this. In particular, the new fragments inherit the original velocity, so they appear to follow the original shot.