So does an empty coke or water bottle. In parts of Europe it’s common for suppressors to be used on hunting rifles to not disturb people who might either be or live within earshot of a non muffled rifle and it also helps to not spook the deer they are hunting; particularly if there are other hunters in the area.
Yes, they should be legal, and without any kind of extra paperwork. It’s just another firearm accessory.
A silencer/suppressor should not leave any markings at all. In a perfect world the bullet itself would never touch the inside of the barrel even let alone the silencer after the igniter fires the powder. The gas from the powder exploding pushes and surrounds the bullet. Still, since we don’t live in a perfect world, the bullet will rub against the rifling in the barrel and leave the same marks bullet after bullet giving it a unique fingerprint if you will. Over time the rifling will wear out rendering the barrel near worthless for accuracy.
In this way the firearm can still be accurate because it is consistently exiting the barrel the same way every time for the most part.
FYI: The longer the barrel the higher the velocity because the powder and gas have more time to push on the bullet as it moves forward. Once it leaves the end of the barrel there’s no more push which is why the same size caliber in a rifle can do more damage (due to higher velocities) and go farther more accurately than a short barreled handgun in the same size caliber.