A militia is a military force that is raised from the civil population to supplement the regular army in an emergency. Therefore, it seems that, if they were going to be fighting invading nations, they would need the most advanced weapons to fight said armies. As you stated, we can't go by what was available at the time, which I agree with (that was a great point). So, the authors must have intended for the civilian militia to be armed with weapons that could keep up with invading, modern armies, whatever that might necessitate. This would include all conventional weapons (and possibly beyond). This is why I don't think original intent is a reasonable way of limiting the meaning of the constitution. We have access to a great deal more information than the authors of the constitution did.