Well, maybe the 'cartridge'/case is ablative material/metal to burn off or sharpen into the spike when the round is accelerated to hypersonic speeds, explaining both the shell casings and the fact the gauss rifle uses bullets? I mean, I recall the current railgun the Navy makes has a giant-ass muzzle flare...
Or given they like belt-feeding weaponry (For changing out the ammo), maybe the casings are mostly just to hold the spikes in firing positions as opposed to being filled with delicious gunpowder?
Although, given they do have different ammo types, they might have a 'bullet' so they can choose what -type- of spike they're firing, I guess...?
Or maybe they just use a different weapon in cutscenes that has normal boolets?
*admits to grasping at straws*
I mean, I'm pretty sure the main reason is 'because the gun looks and sounds cool' and then they decided to come up with a reason for it later.
xD
Frankly, given Terrans Gonna Terran, it's entirely equally plausible to my whimsical headcanon that they game up with a gauss rifle that works perfectly well without needing shell casings only for the general in charge of the product to demand it fire shells 'because shell casings look imposing on the recruiting videos' or his uncle owns the largest ammo manufacturer on Tarsonis or something. xP
Cue vaguely frustrated designers going back to the drawing board....