Mark just an idea. I made a tool out of a brass rod that allows me to insert into a barrel and feel visable pitting. It does not measure how deep, but you might be surprised how swallow pitting really is that looks bad.
You can detect with some degree how sever the pitting is. Dean Harris with SKEETS Gave me this idea.
I am sure you could detect a crack using his tool. Also with out too much work you could build a vaccum or pressure tester. Even pluging the end of the barrel and pouring in some solvent should confirm a leak I would think.
It just seems strange the crack would be so perfect.
My experience where I have seen corrosion along an aircraft stringer where it meets the fuselage skin, the line although it ran horizontal fairly stright. The resulting crack was jagged and very porous.
http://parkerguns.org/forums/attachm...1&d=1518493051