Ordnance Sgt's were officially part of the Regular Army. Regimental Armorers were a different beast altogether and were sometimes present in a role similar to an Ordnance Sgt but at a much more... lower level. they would have been in more charge of small arms repair and maintenance at the Regimental level whereas an Ordnance Sgt was at a higher level such as Brigade or higher. There is some evidence that State units would appoint a Regimental Armorer as an Ordnance Sgt on paper but their uniform appeared to be no different. I've never found evidence of a state or volunteer unit having the Ordnance Sgt insignia as an actual insignia. An Ordnance Sgt was tasked with more than just the small arms. He was assigned to the post more than to the unit and was in charge of the arms at that post. Typically the men would be issued arms from the post, not to them. So when they left to go from one post to another they would not be expected to take their arms w/ them unless going on active campaign. The Ordnance Sgt would stay at the post.
Ordnance Sgt's were somewhat of a beast of their own with authority not unlike that of a Sgt Major, in some cases they answered only to the Post commander or even higher. Most were long serving NCO's w/ real time in grade behind them. As to the star, I've only ever seen pics of them w/ the star point up in the same way as that of a flag. Inverted... I can't recall ever seeing that in a pic. That said I don't recall seeing an Ordnance Sgt rank pictured except on a Frock or Sack Coat. I don't think I've seen on one on a mounted services jacket or Lt Arty jacket. Yes an Ordnance Sgt looking at Arty ammo would make some sense, he was after all responsible for all ordnance to include arty, ammo, small arms etc.