The resort to ricin-tainted letters and death threats says something, I think, about the quality (really, the lack of quality) of arguments against universal gun registration. While all other rights in the Bill of Rights are subject to balancing against the needs of public safety - for instance, you can't cry "fire" in a crowded theater (unless there actually is a fire) - at least some gun-control opponents decry any balancing of the Second Amendment's right to bear arms. They want it to remain as unbalanced as they themselves appear to be.
I wonder whether Wayne LaPierre and his fellow gun salesmen at the NRA will speak out against attempts to kill President Obama and Mayor Bloomberg and death threats against legislators. If not, we can only assume that they consider violence and threats of violence as legitimate forms of argumentation.