the amount of times I have Magnafluxed things that have been welded you always see the edge of the weld regardless of the qaulity of work I will have to take my H Grade Lefever barrels to work and try it and see what it looks like I would imagine a series of radial marks following the twist I will have to try it this week and report back
dont forget with a crack test you have to apply the charge across the crack it will show up on a parrell charge but it works much better across it at least in my field of work (engine reconditioning)