To cut way down on clutter, can you define a variable your Lua script uses but can be set in an ini? For example, if I want a Lua function on three different guns to have three ranges without having to triplicate the code just to change the ranges.

No, but you could have the same script run by all three guns and have the script check the presetname of the gun.

Or better, have it defined as a multiple of SharpRange or something so you don't have to do a pile of ifs in an attempt to cut down on clutter.

Ah, so if presetname = X, then variable = y? That's only an extra line of code, cool.