I'm not sure if someone's touched on this already, but it's not likely that Minmatar projectile ammunition resembles Earthbound projectile ammunition. First of all, combustion requires oxygen and, since there isn't any in space, you'd have to pack the oxygen with it. Odds are the expanding propulsion fuel of the round isn't an explosive as we'd know it. Second, a very, very fast round usually scrapes 4,000 feet/sec (1,249 m/s) and that's a slim rifle round. Step up to an M5 Browning heavy machine gun, and you have an exit velocity of about 2500 f/s (849 m/s). As the caliber goes up, the speed of the round falls, as the propellant has to push more mass out of the way. Once you're into heavy autocannon territory, and you're calculating your range in kilometers and speeds at 300 m/s, that means that even in space the rounds simply are going to go too slow to accurately predict. Third, though caseless ammunition would be impossible with a conventional round (since you'd need to carry the gas with you), even cased ammunition doesn't make much sense because a bullet requires expansion to drive it out of the barrel. Without a readily available gas, it's hard to get that kind of expansion. The icon definitely shows the rounds as being cased (which would make sense), but it's hard to imagine that pumping out shells around your spindly ship inside the shield that stops enemy rounds from hitting you wouldn't interfere with your other systems.
We're sort of overthinking this, though. Amarrian lasers obviously have some similarities to lasers we have on Earth, but we haven't developed frigate-sized lasers that can punch holes through metal from that far away. They are likely somewhat similar in design, but are using very different technology than what we've developed.
I'd imagine that Minmatar projectile rounds are probably using a very different propellant to what we're used to and we can't really say whether it is caseless or not, but I would imagine that it wouldn't be ejecting anything. I'd say it'd be easier to just keep to the basics, that Minmatar projectile weapons are probably using a very different propellant than what we use (plasma?) that consumes it and any casing it might have. More than that, we'd have to assume that it carries all the elements that it needs to work in a vacuum with each individual round or that there's some kind of inexhaustible tank of propellant additive that sets a round off somewhere in the gun or ship. We can also assume that the calibers are fixed in a few very sizes or that the round, if not a railgun, is at least using a magnetic coil to keep it centered in the barrel (which might make sense if the round is moving quickly enough to work in a range of kilometers, as this might eliminate wear on the barrel). Still, it would make sense if the rounds were only a few fixed calibers meant to work in most guns. After all, when you're in space in a ship, there's no problem with air resistance and relatively less issues with ammunition weight and storage. Changing the size and shape of the round by a millimeter or so isn't going to make an awful lot of difference, so the idea of simply standardizing a set of bores tailored for how big a ship needs to be to fire a round without the recoil doing as much damage to the firing ship as the target would make sense. The gun would probably make a lot more difference in space than the round, so unless you're making your own ammunition, I'd imagine even civilians with projectile weapons are probably using standard military calibers, U.N.O.
The guiding principle seems the same, though: getting a relatively small lump of metal moving quickly enough to punch through hardened armor and hopefully hit something vital. That's really what's important.