My guess would be that a bulge at 18" has an Ivory Soap percentile (99.98% Pure) chance of being an obstruction. I have seen some reports, which appeared to be verified, of a bulge or burst occuring from a phenonimum of the shot charge itself acting as an obstruction. This occured from incomplete combustion of the powder charge getting the shot a ways down the bore & then the burn increasing & the gases rushing down to catch up with the slow moving shot which created the obstruction. This can occur with the first shot from a verified clean, unobstructed bore. Some feel this may well have been a problem with the early smokeless powders which didn't ignite as easiy as had black, but were often loaded using the same primers. Smokeless powder in Damascus of course received the entire blame.
This is one of the reasons I am not a proponent of very low pressure loads utilizing slow powders behind light shot loads.
This however would normally occur much closer to the breech than 18", so I would still opine an obstruction.
