I’ve tried looking through a couple of threads trying to find the answer, but they all seem kind of confusing. If all I want to do is make it so that Tranq Arrows do 0 damage, but still retain the same Torpor effect, do I have to change values only in the PrimalItem file, or in the WeapBow/Crossbow/CompoundBow files, or a mix of both? Which values in particular have to be changed?
Neither.
The damage done by the arrow is defined in its projectile file.
However, be aware that by default, torpor equals a % of base damage. If you want it to do no damage but still apply torpor, you’ll need to set the value to an absolute.
There is an option in the projectile to do no damage, tick that 
I have recently looked into a tranq gun that doesn’t do damage. If you want to maintain the exact same torpor amount, keep your settings the same and just find the projectile of the arrow you are copying, copy that and just tick the option within the projectile do no damage. Then point your arrow at the new projectile.
If you need any more help, I can post screenshots.
Will the torpor still be given with default settings (2x damage + 2.5x damage over 5s) if you set it to do no damage, as opposed to setting the damage to 0?
You can always test it and find out. I’m waiting for the 218 kit to work on my tranq mods.
Tranqs torpor work off of %'s… So if you tick ‘do no damage’, you well get 0 torpor effect. At least that is the way it worked ages ago when I made my tranq stuff.
And for Torpor %, its a multiplier (like everything in the game) that is based off of the base projectile damage.
So if you want to do no damage… You can’t. You need to have a min of 1 damage.
Then you can multiply your torpor by what ever value you want your torpor to come out at.
E.g. I made my tranq darts do 400 torpor. At base damage of 1, they did 400% torpor.
Not the case anymore. I made a weapon for fun with a multiplier of 99999 on the projectile and a damage of 1 on the settings. Within the projectile settings I had Do No Damage ticked and it still KO’d the dinos instantly.
Sorry for the late response as I’ve been out of state, but thanks so much for all the help guys!