There is a term called "flash rust" that appears on unprotected steel surfaces in humid enviornments or say when the oils/acids on ones skin causes rust to show up in the areas that were touched. This is a surface rust and can be removed easily. If left untreated, it can develop into pitting, but that would take a while. Flash pitting is probably at made-up term that describes what the OP has described. There's other erosion symptoms that occur in the chamber throat and gas erosion that occurs in the top strap of revolvers and such. Being localized at the muzzle or as Shattered describes, it sounds like there may be other factors involved to cause it to be localized and actually pit the metal. Probably only way to really tell is under high magnification.