A colloquism from the American Frontier days, referring to a handgun with a long barrel, as mentioned in the Marty Robbins song, featured in the video game Fallout: New Vegas. It was the style of the
time for many to carry shorter barreled guns, often with the front sight filed down or completely off, to aid in a faster draw. So, anyone carrying a long barreled
gun was assumed to be confident enough in their ability to handle the
gun they would trade the
speed of a shorter barrel for accuracy of a longer one.