This isn't all that surprising. Any time any kind of gun regulation is considered, the NRA takes the position that the proposal, no matter what that proposal may be, is the first step towards confiscation of all privately owned firearms. The premise behind that argument is that the proposed regulation itself is not enough to motivate NRA members. That's why the organization has to resort to a slippery slope argument. If the proposed regulation was offensive to NRA members on its own, there would be no need for the organization to argue that it will inevitably lead to something else.