True for a couple different reasons
1-One is the bullet could pass cleanly through a body and end up very far away in the woods somewhere and it is never found. Or lodged inside a tree or in the ground somewhere and is never found or spotted by anyone.
2-a bullet ("slug" or projectile) may get very deformed or even broken into small fragments as it hits bone and richochets around inside a body. Which could make the deformed bullet or small fragments almost impossible to compare to anything.
When we typically think of comparing the bullet projectile part of a cartridge we typically think of LE recovering the suspects gun and then test firing another bullet into a barrel of water to compare to bullets recovered from a body. The bullet from the water barrel will give a nice round in tact bullet with markings from the guns barrel. However a bullet from a body may be in pieces or deformed so badly that it makes comparison almost impossible. We can only hope with some luck that LE gets what they need from crime scenes to solve cases.
In this case if LE can recover two bullets from two different crime scenes they could prove the same gun was used even if no gun was recovered. That is if the bullets are in good enough shape. For example if they find one bullet from the Indiana crime scene and one bullet from the Colorado crime scene they may be able to prove the same gun was used in both crime scenes and not even need the gun to know that.
A lot of us use various terms for the parts of a cartridge and its acceptable slang in most cases since sometimes it doesnt matter. When dealing with forensics though then it becomes important to be sure we use the part of the cartridge that LE is testing. Because they can test many parts of the cartridge.
1-LE can test the shell casing for marks that a gun's ejection system does to the shell casing if the gun is a semi-automatic. The shell casing is a lot of times made of brass. A revolver does not have an ejection system like a semi-automatic and rather has a manual way to pour out the casings or remove the casings from the revolver. Revolvers are not as easy to compare markings on casings to as compared to a semi-automatic which typically leaves scratches or marks on the casing as the gun ejects the casing out. I had one semi-auto that left terrible scratches in the same place on every shell. The naked eye could tell the gun was used to throw the casings out because the marks were always the same on every shell casing.
2-LE can test the bullet (projectile) for marks that a guns barrel does to the bullet as it exits the barrel.
3-LE can test the primer for marks that the gun's hammer does on the primer as the hammer falls on the primer which begins the firing of the shell.
4-There may even be more things they can test so it gets pretty interesting.
To remind us of the four parts and proper terms which most of us are already familiar with here.
A cartidge has four main parts.
1-Case or shell casing = holds the primer, powder, and bullet
2-Primer = usually the small inner round thing that sits on the end of the shell casing that contains a small amount of explosive that explodes to cause an ignition of the main gun powder which is inside the shell casing.
3-gun powder is what fills the shell casing
4-bullet = the projectile that gets fired out the end of a gun. Usually lead or some other type metal or metal alloy.
https://www.hunter-ed.com/washington/studyGuide/Parts-of-a-Cartridge-and-Caliber/20105001_700046700/
