If you are changing the projection texture (gunlight.tga) while doom is running, you can type 'reloadImages' to see your changes reflected immediately. Handy for testing.

Unfortunately, while 'reloadScript' will cause your scripts to refresh, but will exit the current game to do so.

addition:
'reloadDecls' will reload your material (.mtr) data without exiting the game, handy for testing your light scripts.

note that while reloadDecls will change your projected images in realtime, if you make related change to your .def files, you will have to swap away from the weapon and back again to see the changes (eg nozzleglow).

Yes, sorry - the pk4 file should be extracted. My mistake.

Further, if you want to test .script and .def changes quickly, add this to your config:

bind j "reloadDecls; reloadScript; map testmaps/test_box; give all"