Assuming 28" barrels, you are talking about a crack that would be around 22" long. It just seems highly unlikely that you could get a crack that long in pattern welded barrels without having it actually visibly open up somewhere along the way where pressures were higher. I've checked for cracks in engine blocks and heads using the poor man's Magna Flux method of wetting the suspect area with gasoline. The gas evaporates quickly on an unbroken surface, but remains wet longer where it seeps into a tiny crack. But being so close to the rib makes it likely that you would just find open areas in the solder joint. However, you might be able to plug one end of the barrel and fill it with gasoline (safely- outdoors of course!) and look for any seepage to the outside. Gasoline has a much lower viscosity than water, so it will seep through a very small hole or crack easier.
A safer way might be to plug and pressurize the suspect barrel and submerge it in water to see if any bubbles emerge from where you think you see a crack.
