Bombs aren't that easy to build and aren't as easy to control the outcome of the attack. But if it were so easy to build bombs, why aren't there more bombings as opposed to shootings. Simply because to the availability of these types of weapons.
In my opinion, you've brought up two different and distinct issues. The availability of guns and the crimes those guns allow for in the hands of a criminal, as well as issues of education and mental health. Not all gun crimes are committed by the mentally ill and not all mentally ill commit gun crimes.
So to help lower mass shootings, limit the sales of these firearms. Its true that criminals will always find a way to do what they intend to, but that doesn't mean we should make it easier for them.
RE: Reality Check: Trump Did Not Make It Easier for Severely Mentally Ill People To Buy Guns