For the story I'm coming up with, there's two planets in a binary orbit, close enough to be travelled with early spaceflight. Over time, both planets are inhabited, and due to evolution, plants on one planet emit roughly half as much oxygen as the other, processing it to be more energy-efficient. This leads to two roughly earth-sized and shaped planets, except planet 1 has an oxygen level of 25%, and planet 2 has an oxygen level of 16%.
Now, with two different planets, there is a rebellion on the lower-oxygen planet as a sort of "revolution" from Earth. What I'm wondering is if gunpowder weapons will be altered by the lower oxygen concentration. If the explosion size is different, then by how much?
I know from a quick explosion search that high explosives are unaffected, but gunpowder is a "low-explosive" and thus burns.
The firearms used vary from muskets and cannons to modern firearms.