Shell Casings:
Are they so unique to each gun that LE would know (although haven't released) what type of bullets (gun) were used to kill the girls?
Hi,
All firearms & ammunition can be traced back to a source. I'll try to keep it short but enough to let you know why it's so important to Law Enforcement.
The mechanical actions of any firearms will leave unique markers on the type of evidence being looked at. The caliber would be the very first thing known if the casings are left behind. It will also tell the L.E. what type of firearm was likely used. If they are left behind, they almost assuredly come from an automatic or semi-automatic weapon. These types of firearms use part of the propellant gases to actually work the weapons mechanism to eject the spent shell & then chamber the next round. This would be true for a handgun or a long arm (rifle, shotgun, etc.) in both the auto & semi-auto class. It would be true for any long arm that does not use the gases for an automatic ejection & chambering too. A lever action rifle would have to be worked manually to eject the last shell & chamber the new round. Same for a bolt action. Most of these types would leave the casings behind in the case of multiple rounds being fired at a short stretch of time. If they fire it just once & don't work the mechanism, the empty shell goes with them & the weapon. Shotguns come in all types of variations. You have single shot, double barrel, and bolt, semi-auto & so on. They would normally leave the shell casings except for the single shot & the double barrel. You would have to manually open the mechanisms on both, pull out the spent shell & then reload. A revolver (handgun) that normally holds six shells will NOT eject the casings. The action just turns the next round in place & all the casings remain in the weapon until manually ejected. Normally, a shooter fires as many rounds as possible & then books it. They would carry the casings away with them in a revolver's mechanism.
All casings can be picked up by the shooter(s), remember that. The mechanisms on all firearms will leave a unique ejection imprint on the spent shell. This can be traced & matched to the weapon that was actually used. The shell itself will have a unique imprint from the firing pin from the actual firearm. This can be traced & matched too. Rifling is a method where there are lands & grooves formed in the making of the firearm's barrel. Just remember a James Bond movie, it shows lands & grooves very well at the beginning of the film's opening sequence. These actually start a long arm's or handgun's bullet twisting to stabilize it in flight. Each weapon will leave a very unique marking on the bullet & that is unique as a fingerprint. In autopsy, they would recover the spent slugs & keep them as evidence. If they have not been too deformed in the shooting process, these can be matched to a recovered weapon. L.E. would just send both to forensics for testing. They would fire the weapon in a water tank, recover the slugs & then microscopically check the lands & grooves on the evidence slug against the test slugs. They will tell if it matches the weapon or not just like a fingerprint. With a shotgun, you are in a different field. The shot travels down the barrel in a plastic wad cup to protect the shot. When the shot leaves the barrel, the wad cup falls away. The autopsy will give them an idea on what type of gauge & how far away was the shot. There are different size shot & they will spread per distance traveled. If they leave the spent shells behind, they can be matched due to markings on the brass from ejection & the firing pin on the firing cap. If they use rifled slugs or rifled barrels, you are back in the same park of matching a spent slug to a specific weapon. Not as easy as a handgun or rifle but it can be done. L.E. can also use the spent slugs to match recovered ammunition from a shooter. Obviously the caliber has to match but they can also check the spent slugs chemical make up to the recovered ammunition. They can get very specific on when & where the ammo was produced. The spent shells & the ammo have to match as far as the manufacturer obviously.
Hopefully that is not too long, but enough info. Feel free to ask any questions. If I think of anything else, I'll post it.
