I've almost got Sharpshooter (and, by extension, Great Weapon Master) nailed for scripting the mechanics. You will need to check "Show in Activated Ability List" and set an Activation Name.
Code:
doneif (field[abilActive].value = 0)
foreach pick in hero from BaseWep where "(wCategory.RangeProj | wCategory.RangeThrow | wCategory.Firearm)"
eachpick.field[atrBonus].value -= 5
eachpick.field[dmrBonus].value += 10
nexteach
Now, when you have Sharpshooter selected, a check box will appear on the in play tab. Checking the box will change the attack/damage values automatically. It does not, however, test for proficiency - not sure how to do that yet.
For Great Weapon Master, replace (wCategory.RangeProj etc) with wProperty.Heavy, and use atmBonus and dmmBonus.
Once the Weapon Proficiency test is sorted, it should hopefully slot into the where expression - but for now, it'll do - just hold the honour system there.