That's very true. However, guns are not flowing into the US, but in the other direction out of the country in huge numbers. The real question is whether we want to seal out borders. We could certainly seal our borders and stop the flow of guns, if we wanted to do that.
"Can we put preventative measures in place to at least hinder nutjobs from having guns...sure...but once again...there will be holes in the system."
Again do we want to close those holes or not? Certainly we could close most of the holes, if we wanted to do that. We could make it very hard for crazies to get guns, if we wanted to do that.