Ok, in my gun the barrel is not fixed, so I can swap it for another one in 10 seconds. So what if I have two barrels, one I use only when I go to ballistic lab, and another one I use every day. Now tell me how their modern system can help to identify me by the bullets fired from my gun?
The shell casing can be matched by several marks produced when shot from any firearm, including a shotgun.
As you noted, you can swap the barrel on some guns, but hardly can you replace the receiver itself as easy.
Again, the swapping of barrels on a gun in most cases leave behind marks from the tools employed, in the case of revolvers that can be done so. For this very reason is why the DR employs a new marking tool on the main parts of all guns undergoing the process. That's to say not only is the bullet marks recorded, but also the marks produced by the firing pin, seating, ejector, receiver, etc... on the same shell. Added to the new marks placed on the gun parts as noted.
Yes you can swap the barrel, but the bullet casing will still leave behind a clear print of the gun that fired. In the case of a gun that was used to commit a crime and the barrel later swapped, the swap will be easily detected as the bullet marks fired from the new barrel will not match the old print. Unless you say that a person will swap the barrel, etc... prior to using a registered gun to commit a crime? Pretty stupid and highly unlikely as noted by the reason of the ammo casings.
You can get a robust profile of a shotgun's spent case as well. As noted, the marks left behind from the receiver will be there.
In the case of the DR, it simply helps the Police to know if the weapon used to commit the crime was registered or illegal. This allows the investigators to focus on other parts for clues in finding the authors of the crime.
The news of when we got that specific marking tool for guns was published a while back in the DR media. Most if not all of you missed the tiny report as usual.
Also missed the press release of the FBI assisting the DR Police in setting up the lab and databank, linking both ours and theirs.
The ballistics are taken, but also marks stamped onto the most essential gun parts at the same time.
The annual check is to keep the database as updated as possible since wear and other factors play a role in changing characteristics of the guns. A check also goes on to see if essential parts have been changed on the guns and noted.
Like I said, the FBI would love to have the ease of access and rules like the DR can when it comes to guns and how to gather data on every single one in the hands of the citizens.