securing bad guy's gun at the scene
If you shoot a bad guy who comes at you with a gun or any weapon for that matter, you want it retained as evidence you were justified to use deadly force.
I have heard that sometimes shootings attract a crowd and members of the crowd will be only too happy to pick up a gun or knife from the ground.
Naturally, you want the weapon there when police arrive, but there is concern about tampering with a crime scene. But then again you don't want bad guy, who may not be dead, to pick up his weapon and start firing. So that legitimate concern, as well as securing weapon, may override any concerns of tampering with crime scene.
Look forward to your advise. Especially interested in if or how you would secure weapon in various situations. Jack