Well then how about this link as a foundation.
This is a link to a news article listing all of 2012's mass shootings defined as
multi-victim shootings where those killed were chosen indiscriminately.
Now I spent over 8 hours researching each one going right down the list to determine which ones actually qualified under the definition above and collected many little details as well, such as were the guns used legal purchased, or were the gunmen prohibited possessors, and of course would any of today's legislation like background checks for personal firearms sales have made a difference or even come into play.
I made many notes, I have a text file I have attached with the notes, but alas I was well into the document when I figured maybe I should have noted the links to the info. Alas, take it for what it is, unproven documentation from unproven sources but, it isn't that inaccurate.
Here are my analytic notes;
16 Mass shootings resulting in at least one fatality in 2012.
Despite the articles claim that victims were selected randomly or indiscriminately it is obvious that 9 of the case were neither. The victims or target locations were selected for a specific reason.
6 of the 16 may have involved an AR-15 style semiautomatic rifle but some reports had no evidence for certain.
9 of the cases involved legally owned firearms.
6 of the cases involved stolen firearms.
1 case involved a prohibited possessor who "legally" purchased his firearms before his status could be updated in NCIS so his check went through clean.
4 cases specifically involved a prohibited possessor, at least 3 others should have, but at least 3 cases were specifically gang or criminal in nature.
Of these 16 cases the Background Check system only failed once and a new Background Check requirement on private sales may have had an effect on one case[England/Watts]
The headlines read "leaving at least 88 dead". The numbers break down to;
Dead Assailants 10 with 6 killing themselves, 4 killed by Law Enforcement.
85 actual slain victims including slain Law Enforcement.
But if you tweak it down to only those events where the shooting might have been effected by background checks or a beefed up mental reporting system where troubled people are spotted and flagged more effectively, and rule out their ability to still obtain weapons illegally, the numbers are more like this(I left the Sandy Hook incident in as well);
Seven shooting incidents.
Dead Assailants 6 with 4 killing themselves, 2 killed by Law Enforcement.
45 actual slain victims including slain Law Enforcement.
Grand total of all fatalities is 51 and 27 are from Sandy Hook alone.
Of these seven cases, four might have been prevented by better mental health reporting procedures. One by faster NCIS updates of prohibited possessors and one perhaps by Private Sales background checks, and one I don't think could have been stopped at all, the laid off Accent Signage Systems employee.
So as far as background checks goes, we have one case where closing the loophole might have had some effect, [England/Watts] Tulsa hate crime shooting, three black people killed, randomly selected, but not truly random because the target selected was the entire black race.