Is that really it? Just because of somethings = dieselpunk? There are also alot of mechanical explenations to these guns in a steampunky nature.
Seriously, theres more steampunk than dieselpunk in visual explenations. The major steampunk elements are quite major.
Like you mentioned where guns have dieselpunk elements, yes thats true. But that is to make some weapons believably possible and not rediculously stupid.
Mercury to me being the most advanced looking steampunk along with gattling, while the carronade or hwacha or any heavygun to dieselpunk.
Dont dismiss the air-ships with baloons. Not to mention how they are set up. Its extremely obviously mechanical. The engines are fueled with diesel or whatever, but they arent jet and also they are exposed rather than insade of a shell for crew to easily fix and mantle with it. Which is my primary reason for it being steampunk. Not gears, gears are there to show how something works, and they arent incased but rather exposed because in a world of steampunk, everyone knows how to make anything with the use of mechanical engineering. How the ships are made, look and how they work with how the crew works with the ship is more steampunky than dieselpunk. Dieselpunk is alot more solid and steadfast with their technology, closer to an enhanced world war 2 set of tech and its time period.