There is a method out here used by a company called Vetco ,its used to inspect high pressure pipe ,and casing.
It uses some sort of magnetic principal .
The whole thing is mounted on a truck making it mobile.
One of their guys used it on my tubes once, and he found nothing ,but it was used on a 3/4 scale tube belonging to another guy, and they found laws in the casting.
The 1/2 in .liner was OK though ,and he had fired the gun a lot ,so i guess he just left it alone.