Just to let you know a little bit - when Union military units were first formed in Missouri early in the war, it was usually by a prominent person in a county and he provided weapons and sometimes horses. After the Federal government realized the importance of Missouri, they started funding the military. These local groups were rolled into an 'official' regiment.
So I would say that 'Argus Rifles' was a unit formed by a guy by the name of Argus and he donated the weapons. It was later taken over by the 6th MO.
You can go to the following links and probably find a bit more about your relative:
(He appears to be a little young to be in the war as he would have only been 5 when it started and 9 when it was over)