From the Hallowell site's Firearms Dictionary:

"Rivelled - A shotgun barrel damaged by having a series of slight ring-bulges or wrinkles. Caused by heavy loads or the reflected shock from a minor obstruction in the bore. Best detected by viewing along the outside of the barrel at a very shallow angle against a point-source of light. Very difficult to repair satisfactorily."

May God bless America and those who defend her.