It's for ideological purposes, which is why you have a massive difference between their figures and somewhere like the bbc/sky... 11 for MJ and over 400 for UK media reportingYou'd think it would be fairly straightforward to count up fatalities through firearm (with death cert showing homicide/ suicide/ accidental/ other).
Then count up hospital first admissions for firearms injuries.
Both recorded with time of incident (where known) and healthcare facility of attendance/ location of certification.
Set definitions- single shooting, multiple shooting types (breakdown as required- e.g. 2-3, 4-5 etc).
The fragmented, and private, nature of US healthcare and differences in state law probably makes that all rather difficult...