Who do you report to if you know a felon who owns a firearm?

In general, you should call the police or sheriff (who ever it is that has jurisdiction were you live) and get an officer to take a report - if you don't want to go in and file one. With the police report in hand, the officer will decide what action to take. You will be asked how you know the individual, how you know he is a felon, how you know he has a weapon, and all that. A felon having a firearm is serious issue, and serious and probing questions will be asked to pursue the investigation and assist the officer in deciding what to do next. You may feel a bit like you are the suspect, but if you are sure and you believe you are doing "the right thing" by reporting it, stand up and make your statement. And recall what Joe Friday was always saying. "Just the facts, Ma'am." Just give the facts. No opinions are required unless asked for. Let the officer give the opinions - like what to do once he has your report. Review the situation in your mind and then make the decision whether or not to make the call. You have the duty to "do the right thing" here as a citizen, but it is up to you what you consider is the "right thing" in this situation. It's that simple.