The rimfire brass is like a miniture of the old balloon head revolver cases. If you made it rimless you would loose a significant amount of case capacity. To compensate for that, you'd have to make it longer. That would make it hard to fit in handgun grips and reduce the amount you could put in a tubular magazine. Actions would have to be made longer as well.
Proud member of the NRA and Texas State Rifle Association. Registered and active voter.