@OmSigDAVID,
In a porous system, with too many firearms already in circulation - it's not possible to prevent a mentally ill person from obtaining a firearm...but it is harder. Say Colorado was indeed a nutbag, with no prior criminal history...so criminal connections probably isn't his thing for obtaining guns - he otherwise goes on the internet (which would trigger the same control regulation, if it was done properly) or attempt to do it the legal way.
One the issues the US faces (if it were to consider uniform federal level gun control laws) is the lapse time it would take for them to become effective, which may be several years to a decade. It would also need to sort out how to prevent illicit importation.