Some pretty good answers and some far fetched. Odd Job did the best.
Here’s what can be done with a shot gun, when they have the suspected weapon and evidence from the crime scene.
They can determine approximately how far the suspect was from the victim. When shot from up close, the gunpowder that it still burning hits the skin and leaves what is called gunpowder tattooing.
If there are empty hulls at the scene, they can be matched to the gun.
The gun leaves marks on the hull. The extractor mark, the ejector mark, chamber marks and breach face marks.
The breach face marks are the most important and are left by different things. First are the tool markings left by the cutting tools. Then there are shear markings from the firing pin hole in the breach face. Most of the breach face marks will be located on the primer.
This is how those marks get there. When the gun is fired, pressure expands the metal portion of the hull. When the hull is extracted the chamber will leave marks on the metal sides of the hull’s rim. The extractor will also leave a mark. If the gun is a pump are auto, as the hull is extracted it will Hit the ejector, which will leave a mark.
When fired the hull is also pushed back onto the breach face. This will transfer any tool markings.
Here’s the most important part, when fired the primer is pushed back. Some of the primer gets pushed into the firing pin hole. This is called primer flow back. If there is any movement, up, down or sideways, the firing pin hole hole will shear The flow back leaving marks. The firing pin will also leave marks.
I, myself, do not deal with shotgun shells, just rifle and pistol cartridges. Here are some of the pics I take and look at.
This is the firing pin impressions from a Glock. Two different cartridge cases that were fired from the same gun.
This is a 3D pics
Here aer the shear marks matched up.
This is from a different gun.