Based on Kaihlik's 'Ammo Weight' topic I've created an excel tool that calculates the weight of ammo. Since the topic was never finished I've made some extrapolations and generalizations, e.g. in case of bullets I've added 2g per +1 average damage, which is consistent with Kaihlik's findings.
The short of it is that this results in a much more consistent ammo weight for the ranged weapons used.
The tool can be downloaded here