More than you probably want to know about Magnetic Particle Inspection (MPI)
https://www.nde-ed.org/EducationReso...principles.htm
VERY few testing labs have experience with pattern welded tubes, and interpretation of esp. Twist barrels can be difficult. A normal exam however would be reassuring
MPI is generally considered to be the best method for the detection of fine, shallow surface cracks in iron and steel, especially fatigue cracks. However, defects deeper in the material ie inclusions, may be missed.
It of course tells you nothing about the bore, and for that reason direct observation with a bore scope, and wall thickness measurements of any vintage barrel are needed prior to use.