What are some subtopics for a topic "Should U.S. citizens be allowed to bear arms?"
If you are writing this as a research report or essay there are a couple of subtopics I can think of that you could write about. They can range from the history of the Second Ammendment to arguments for and against it.
I think an important subtopic, and the one that should probably be first is the history or text of the Second Ammendment. Explain what it says in the Constitution and a little bit of the history of when it was written. Of course, how much history you may want to write might depend on the length of your paper.
You could write another subtopic about the debate on gun laws in America. That subtopic could be broken down to outline the arguments in support of the Second Ammendment and the arguments against it that support stricter gun control. In that subsection you can maybe also outline pros and cons to both sides of the arguments.
A final subjection could be your personal take on the ammendment and what you think it means and how you think it should define gun control laws in America.