I could postulate a number of potential offenses, but the one most likely to be relevant to the situation would be poaching as it hunting out of season or without proper license (since the report indicated that it was a rural area).
Regardless, the officer should have secured the firearm, had a conversation, found out nothing was going on, and then returned the firearm. And all of this would not have been an issue.
Of course, you could.
But the criteria is that you must have evidence. What evidence, aside from carrying a rifle, would allow you to detain Grisham for poaching?
And what would you postulate had he been open carrying in an urban area?
I could postulate you're a kiddy porn addict because you have an internet connection. Just turn your smart phone over to the nice officer now so he can clear you of any wrongdoing. You have nothing to hide, right?