The problem with this whole line of reasoning, in my view, is that the laws I find most objectionable are usually the ones that are most stringently enforced. For example, if you or I was caught with an unregistered NFA weapon, you can almost guarantee that we would be prosecuted in a heartbeat even if we'd committed no other crime. Likewise, if you or I was convicted of a non-violent felony like counterfeiting, it would still be reported to NICS and we still wouldn't be able to pass a background check to buy from an FFL.
The existing laws which are not commonly enforced usually fall into one of two categories: they're either so poorly written that they're extremely difficult if not impossible to enforce (922r is a good example of this) or they're the ones which might actually prevent violent crime, but enforcing them would take a lot of the steam out of gun control advocates' arguments. An example of the latter is the ridiculously low number of people who are prosecuted for lying on the 4473. I cannot help but wonder how many prohibited persons resorted to acquiring guns through theft, straw purchase, or other such means only after being denied on the NICS check. By refusing to prosecute such people, we're essentially letting them go free to figure out how to get a gun some other way.
If we look at most of the more notable mass shootings in recent history, we can usually find and existing law that, if more vigorously enforced, may have prevented it or at least mitigated the damage. Had it been widely known that straw purchasing and lying on 4473's was aggressively prosecuted, the straw purchaser who provided guns to the Columbine shooters may have been more hesitant to do so. Had the states been required to report mental health data to NICS, the Virginia Tech shooter might not have been able to acquire his guns. Had the drug offenses and terroristic threats made by the Tuscon shooter been prosecuted by local authorities, he likely wouldn't have been able to pass a NICS check to acquire his guns. And finally if the COPS program hadn't been defunded by the current administration, the Sandy Hook shooter might not have been able to kill as many people as he did.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar