The Cake Is Annoying said:
Well, part of the reason weapons were designed to use low-mass, mass effect accelerated rounds was to prevent decompressions in space (if I remember correctly), and it could be that the rifle was made by an armoury that manufactures weapons that are made for forces that'd never have reason to leave a planet.
That way they'd never have the restrictions of the Citadel's legislation of mass effect weapons that way, and using older and cruder techniques (20th/21st century) might be easier to churn out at short notice, in an emergency.
Or it could be they're completely illegal and since the citadel is doing nothing to help earth, the attitude at the armoury is "Screw the law, give the men something that'll put craters in those metal bastards."
Hell, if it was combining a round that size with a mass accelerator it could hit like a WMD, if the accelerator could get it fast enough. Remember the speach from ME2 about not shooting the giant space ship weapons "from the hip" because "you are ruining someone's day"?
I'm sorry, I don't see how that explains why he's ejecting something from his rifle, though.
Unless they had a hybridized design, like a gunpowder blast gets the round started, and there's an ME generator inside the bullet itself which cranks the mass way down, it gets accelerated down the barrel even faster with magnetic coils, and at the last second the round either turns off its field, or reverses the polarity so its mass gets cranked way the hell up.