Anyway, I bet there is many more legal issues to tackle than just usage of gun powder:) Based on your story, you might endanger other people, infrastructure, aviation safety etc. Also, that kind of activity can be quite easily interpreted as some kind of terrorist plot or something. Outer space treaty is the smallest issue in this case. Gun powder can be replaced to some other method so no problem there.
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Anyway, I bet there is many more legal issues to tackle than just usage of gun powder :) Based on your story, you might endanger other people, infrastructure, aviation safety etc. Also, that kind of activity can be quite easily interpreted as some kind of terrorist plot or something. Outer space treaty is the smallest issue in this case. Gun powder can be replaced to some other method so no problem there.