[Lancey Howard]

The FBI has had the ability to read serial numbers that have been ground off with a power grinder, based on the metal deformation in the metal by the number stamps. And they've had that ability for years.

Yep. But what's scary is that anybody gives a rat's ass what the serial number is on a freaking shotgun!?! Maybe it's the same shotgun my Dad carried over his shoulder when he was 12 and walked down his neighborhood street to Pennypack Park to hunt, as passing drivers beeped their horns and waved.

[MHGinTN]

It is usually stolen guns which have serial numbers ground off.