I know I should respond with facts to moronicity this severe, but I can't help myself ....
Does "RobertEarl" have a quarter of a clue how impossibly ludicrous what he poses would be? There are thousands - tens of thousands? - of gun ranges across the US, and 10s of millions of legal gun owners! Search warrants would be required for each gun range, and since the USC requires both probable cause and a description of the evidence sought, few if any judges are gun-hating-crazy enough to issue such search warrants. Posting enough officers to observe a range fully during range hours would be a significant waste of personnel, and the kind of video necessary - whether taken by officers or by unmanned cameras would quickly entail having a vast disk farm for every gun range. Even the most gun-hating political animal of a police chief in America would laugh at what "RobertEarl" imagines is a stroke of genius!
And does "RobertEarl" realize how many rural gun owners have a little corner of their property set up for plinking and skeet shooting?!
As I said above, there are 10s of millions of legal gun owners. What percentage of mass shooters owned and possessed the guns they used legally? Off the top of my head ... the shooter at that school in CN could not own fire arms, legally. He borrowed/stole his mother's guns. The San Bernardino shooters got their guns through an illegal "straw purchaser". The shooter at that church in SC should have been legally barred from purchasing his gun, but screw-ups by state and Federal law enforcement resulted in the pertinent information not being on his record. The Fort Hood murderer could legally own and carry, but not where he did the shooting. As best I know, the Dallas cop-killer owned his gun legally. IOW, a significant percentage - possibly most - of mass shooters could not legally own the guns they used.
Further, even if you ignore that, mass shooters would constitute a smaller than minuscule percentage of the population of legal gun owners.
The bottom line is that "RobertEarl's" "brilliant" proposal would tie up many personnel and vast data storage space - possibly far more than exists and could exist in the foreseeable future - and would accomplish absolutely nothing beyond giving "RobertEarl" the pleasure of sticking it to innocent people he loathes and hates.