Just sayin'...
Not, mind you, that the source of a claim can't legitimately set off warning bells. Particularly in contentious matters, there's a lot of cherry-picking in popular articles like this one (as opposed to peer-reviewed research published in recognized journals). That is to say, this is a fact sheet published by an agenda-based group, and should thus be checked against available validated data if one intends to cite it as evidence supporting a position. That's what I'd do with any such article, regardless of which side of the debate it issued from.
The red flag for me here is the lack of a citation for the source of the data. The standard source for this kind of data would be the DoJ's Bureau of Justice Statistics, and if I decide I'm interested enough, it would be a simple matter to cross reference between that source and the data tables presented in the study.
I almost certainly am interested enough (when time permits...I'm busy and kinda screwing off even by taking the time to post this!), as I've been a supporter of universal background checks for a long time and I'd like some evidence to see if they're proving useful. Establishing a correlation (or lack thereof) between UBCs and a reduction in these shootings would be a great step. It wouldn't be a demonstration of cause-and-effect, of course, as that would require reduction of confounding factors, etc., but even a simple correlation is quite useful: it prevents ruling out a causal connection.