Air resistance is the reason for terminal velocity and without air resistance terminal velocity would not be an issue. It is not just how aerodynamic the shape is though, the mass of the object also factors into terminal velocity and higher mass means higher terminal velocity. If air rifles have been around for a while in nations outside of the Hegemony, then their bullets might be more aerodynamic.
On a different note, I am looking forward to discussions with neighboring great powers as it seems obvious that while overthrowing the Hegemony might still be possible without foreign assistance, it would be much easier with it and I think a MC that was well-read would have picked up likely historical parallels. I am curious whether there are any real world examples of rebellions against a great power that succeeded without the assistance of another great power. I think my MCs would not be very concerned about the ethics of allying temporarily with any other great powers as the Hegemony is (as far as my MCs are concerned) very close to evil incarnate and in any case “the enemy of my enemy is my friend”. As Winston Churchill said, “If Hitler invaded Hell, I would make at least a favourable reference to the devil in the House of Commons.”