You can export all of the current gamevars and their values by using the cheat DNDEBUG (or the console command "activatecheat 24"), which includes the pre-defined gamevars controlling weapon behavior.
Internally the Expander is referred to as WEAPON11, and the default values for it are:
WEAPON11_CLIP 0
WEAPON11_RELOAD 0
WEAPON11_FIREDELAY 3
WEAPON11_TOTALTIME 5
WEAPON11_HOLDDELAY 0
WEAPON11_FLAGS 2
WEAPON11_SHOOTS 2448
WEAPON11_SPAWNTIME 0
WEAPON11_SPAWN 0
WEAPON11_SHOTSPERBURST 0
WEAPON11_WORKSLIKE 11
WEAPON11_INITIALSOUND 0
WEAPON11_FIRESOUND 388
WEAPON11_SOUND2TIME 0
WEAPON11_SOUND2SOUND 0
WEAPON11_RELOADSOUND1 4
WEAPON11_RELOADSOUND2 5
WEAPON11_SELECTSOUND 219
WEAPON11_FLASHCOLOR 1324248
https://wiki.eduke32...efined_gamevars
I was suggesting making an entirely new projectile that would mimic the GROWSPARK behavior, and would allow you to override auto aiming with various WORKSLIKE flags. Basically coding a replacement weapon for the Expander that looks and behaves almost identically to it, but shoots your custom projectile. From what I'm reading in the source code, it's specifically the GROWSPARK projectile that has the auto aim issue, not the Expander as a weapon itself. Unfortunately I don't think there's a WEAPONx_FLAGS option that would disable or override auto aim, but maybe someone more familiar with custom weapons has a better idea.
In a previous comment you asked how to disable the Expander as a standalone weapon, as you are replacing it as an alt-fire for the Shrinker. How are your shooting the GROWSPARK from the player exactly? If you are already handling custom weapon behavior and firing, then it should be simple enough to replace your command shooting a GROWSPARK with shooting the new projectile.
https://wiki.eduke32...efineprojectile