How accurate is the analysis of spent shell casings?

In 2008, "48 Hours" asked Nelson Welch, an independent firearms forensics expert, to help us understand how and why Albuquerque Police investigators were so confident that a shell casing found inside the home of then-murder suspect Ron Santiago was a direct match with four shell casings found at the scene of a double murder.

"48 Hours" took Welch to a gun range where we fired bullets we could use for analysis.

In the video clip below, Welch was passionate about his ability to spot a match and gave us insight into this sub-specialty of matching shell casings: