Is it conceivable that the officer continued being calm and casual and asked Grisham politely to put his rifle down?
His rifle was down as far as it could be. Forcing him to remove it, without probably cause, is against the 4th Amendment.
EDIT: Now, do I believe he's telling the 100% truth? Probably not, but no one in his position was. With what we know, he didn't do anything wrong. The police were in the wrong. If the police would like to release the dash video and audio, I would happily watch it and make my own conclusions. If that means we can clearly see Grisham did something wrong/illegal, then I will have no problem changing my beliefs on this. Without that, we have absolutely no evidence, besides conjecture (meaning...not evidence) that Grisham did anything wrong.
His intend has nothing to do with whether he should have been arrested or not. If he has a right to do something, then he has a right. Whether it's for a political purpose, or a Boy Scout Hike. I want to make it clear, I'm against using OC as a means to prove a political point, however, I would never say those guys are in the wrong and should be treated like Grisham was treated. Why? Because it's their right, and it's legal in their jurisdiction.