I have attached a patch. The graphical arrow collision is not a perfect match for the gameplay one but it should be close enough.
The code uses a Gaussian distribution and defines spread which determines the scaling of the gaussian. The Gaussian will have width Spread at distance 40m from the archer. This means about 2/3 of the missiles will hit a target of size spread at 40m. The spread is linearly proportional to distance.
Issues:
If the entity being attacked dies before the missile arrives then the missile has no effect.
I consider these to be minor
