Between WBEx and the next chapter of Biomechanics 101 I probably shouldn't take on more writing projects, but fun fact!
This figure haunts me because a) still very sexy b) combustion-propelled weaponry doesn't fit neatly into any of my existing headcanons b/c
ugh god the logistics of itLike, if you wanna have a mass-based weapon? You can just make a railgun. A railgun will have the same ballistic properties, and also you can just power it straight from the frame's generator, you don't need to specially manufacture cartridges or gun frames capable of withstanding the ignition and remaining operational. (Also, don't quote me on this, but I also think beyond a certain threshold, between an electrically-propelled gun and a same-size chemically-propelled gun, the railgun will always be more powerful, and anything beneath that threshold is a peashooter.)
Maybe detectability? A railgun driven by a generator will have its own waste heat radiation, while a propellant-cartridge weapon can be fired entirely cold and won't have much lingering radiation other than what its frame absorbs. But that's also not terribly useful since
mecha have their own radiation as well, and cartridge-firing weapons would only ever be used by... well, mecha capable of masking their other signals, and at that point lugging a whole extra gun capable of firing whole extra rounds seems besides the point.
One advantage that cartridges have (the reason I think they still stick around in conventional artillery) is the transferred energy requirement. Ion blasters and frame-powered railguns require a high amount of fuel that could be used for other things, while a cartridges can be manufactured beforehand. And while railguns hit harder for the same energy, they require a lot of energy in the first place. Chemical cartridges are also extremely stable and easy to transport, unlike raw fuel, and since the firing mechanism can be relatively simple, the problem can often be solved by just ejecting the faulty round and moving on.
And then there's of course the ability to have different kinds of payloads, which isn't terribly useful for personnel application, but is probably the biggest reason they persist as artillery weapons. A railgun firing at relativistic speeds is gonna be more powerful, but a lot of times a good old explosive payload will serve the same function just fine with less overall energy consumption.
So they do have a niche, especially in sustained combat situations where keeping energy consumption down is a priority, even if they introduce a complication into the logistics chain. It makes me wonder if most cartridge-based rifles also have a mode where they can be fired from a fuel line using inert ammunition, only it's more "expensive" than firing a blaster would be.