The difference is, full auto guns have limits and bans placed on them while single fire do not have as much. Assault Rifles are not controlled because they are guns; they are controlled because they are capable of full auto fire. In other words, "assault". And no, the ammunition capability is not kept down because of the ammo, but because of spray.
Now, if you can justify why a civilian would need a fully automatic assault rifle with a 30-60 round magazine then I'll be glad to hear you out.