Heck, I work for the Dept. of Health,/Vital Statistics, and we put numbers to every mundane thing you can think of. I've never seen any such statistic gathered, anywhere, myself. It's just not the kind of statistic that is gathered, in that form, even on most police/incident reports, even.
For example, if a police incident report of an otherwise legit self-defense shooting ends up resulting in an un-intended shooting (miss and hit someone, ricochet, over-pen, etc.) it doesnt normally end up being reported or written up as such.The reports usually end up being X # persons shot. An individual officer may include in their notes or even the report, that one or more other folks found to have been shot may not have been the INTENDED victims,but thats about it.
If it results in a death- cause of death's are not listed as "unintended gun-shot-victim" they are listed as simply "gun shot" or along those lines.
So you really couldnt even deduce much more than that, if you searched through what reports are available.