“How do they know the type of holster the gun went into that shot the dirtbag was if they never recovered the gun...?”
Projectile. It’s not voodoo, just simple gun facts. A recovered bullet alone can give you caliber, manufacturer, and depending on circumstances can point you to a specific model of firearm. Case in point, .357 Magnum fired out of a barrel cut for 1:18.75 RH rifling with six grooves? How about with 5 grooves? Borach cut? 9mm with 6 groove polygonal and a 1:9.84 RH twist? 9mm with 1:18.75?
Rate, direction, number of grooves, type of cut, caliber, those facts alone can point you to something as specific as a given model of Dan Wesson, S&W, Glock, etc.
Beyond that, depending on the size and exact configuration of lands and grooves, it’s possible in some cases to tie a projectile to a specific serial number range of a given brand and model. At times a given manufacturer with a relatively unique rate, direction, groove number and cut might have undergone tooling or process changes that resulted in obvious dimensional changes in their rifling that characterize, for example, early, mid, and late manufacture of a given model.
— Broach cut, not Borach.