Active Duty Soldier Illegally Disarmed And Arrested? [Video]

Christoper Grisham, a sergeant in the US Army was, as he claims, "illegally disarmed and arrested" while walking in a rural part of Temple, Texas with his son. They were out to complete a task so his son could receive his hiking merit badge in Eagle Scouts. Grisham was carrying his rifle with him strapped dangling from his chest. He was disarmed and arrested. Was it legal? The incident was caught on tape. Watch the video and let us know what you think about who was right in this situation.

From his YouTube account, Sergeant Grisham says this:

On March 16, 2013, my son and I were hiking along country roads among pastures and fields with my 15-year old son to help him earn his hiking merit badge.  I always enjoy these father/son hikes because it gives me time alone with my son.  As I always do when we go on these hikes and walks, I took my trusty rifle with me as there are coyotes, wild hogs, and cougars in our area.  In Texas, it is legal to openly carry a rifle or shotgun as long as you do so in a manner that isn't calculated to cause alarm.  In other words, you can't walk around waving your rifle at people. I always carry my rifle slung across my chest dangling, not holding it in my hands. 
At about the 5 mile mark of our hike, a voice behind us asked us to stop and the officer motioned for us to approach him.  He got out of his car and met us a few feet later.  He asked us what we were doing and I explained that we were hiking for my son's merit badge.  He then asked me what I'm doing with the rifle, to which I responded in a calm manner, "Does it matter, officer?  Am I breaking the law?" 
At that point, the officer grabbed my rifle without warning or indication.  He didn't ask for my rifle and he didn't suggest he would take it from me.  He simply grabbed it.  This startled me and I instantly pulled back - the rifle was attached to me - and I asked what he thought he was doing because he's not taking my rifle.  He then pulled his service pistol on me and told me to take my hands off the weapon and move to his car, which I complied with.  He then slammed me into the hood of his car and I remembered I had a camera on me (one of the requirements of the hiking merit badge is to document your hikes).  This video is the rest of that encounter.  Up to this point, I am not told why I am being stopped, why he tried to disarm me, or even that I'm under arrest.

So, what do you think? Was the officer right? Was Grisham right?