Actually ... I'm not sure but maybe I was a little hasty earlier. Heston had been suffering from Alzheimer's, and there is at least a possibility that the NRA were using him, which would be very easy for them to do, given the nature of the condition. This isn't to say that Heston didn't know what he was saying, but there is a strong possibility that he didn't know why he was saying it - in other words, it could be that as far as he was concerned, he was just playing a part, making a speech that had been written for him. I don't know whether Heston would have taken the same position prior to the onset of the condition, but it does seem apparent that there is at the very least a strong possibility that at the time of the "cold dead hands" speech, he was not fully responsible for his actions.