I think we have to distinguish between 'mass shooters' who are likely to be mentally ill, bear a huge grudge or have been brainwashed by some lunatic fringe group, and the everyday killings of individuals by Police, gang members, drug dealers and their clients etc. The former do cause large numbers of deaths in one attack, but the latter (I should imagine) are responsible for far more deaths in total.

The first step I suppose is to outlaw semi-automatic rifles (not sure of the correct term, but those that fire lots of bullets in quick succession) and restrict ownership to simple pistols. But nobody would agree to that. Most folk would hang on to their fancy weapons and hide them carefully from the Law. They seemed so proud of their huge guns on that documentary; they were obviously a status symbol and nobody would willingly relinquish that, no matter how one legislates.