I tried to make the mauler shotgun shoot homing bullets, so I linked to the .lua script in the .ini file:

But when loading Shotguns.ini, Cortex Command crashes.
Do you know what´s the problem?

The problem is that your implementation of the script applies the script to the AEmitter for the mauler, not the shot itself.
If you're okay with making all coalition shotguns homing, put the scriptpath line on the "Particle Coalition Shotgun" object.
If you want it to just be the mauler that homes, go to where it says

and replace that with



And, of course, you need to actually have a Coalition.rte/Needler.lua to reference.