With such a light trigger pull, the integrity of the mechanism is suspect. IT is entirely possible that the hammer "jarred off" as you closed the action. This is not quite the same as a slam fire, although the result is.
Generations of shooters were very happy to get a 3.5-4lb trigger pull on a 99 (and many never got one that light, and were still very happy) that weight being considered the minimum safe pull for a hunting rifle.
Here is a word of warning, unless you absolutely HAVE TO, do NOT take apart the rotary magazine!
All else being equal (and it almost never is) bigger bullets tend to work better.