If one is going to worry about inclusions and voids in DOM tubing, he should also be worried about them in seamless, or even bored solid stock. Fact of the matter is, they exist to one degree or another in
all steel. You may be able to find tubing bored from bar stock that was manufactured using the vacuum induction melt process (which removes most voids) but that won't be inexpensive material. And if it's carbon steel, it's still going to rust anyway.
Unless you happen to get a piece of DOM that should never have left the manufacturer (cracked, incomplete weld, incomplete finishing of the welded area), any defect is going to be localized and
very small. It's not going to run for any length along the tube, allowing corrosion to spread to the point where you have a condition that's going to cause it to peel apart like a banana when the cannon is fired. Neither is a small surface defect going to spread very far down below the surface of the steel like a cancer where it can't be detected visually. It's going cause what any rust does; a big ugly pit.
That said, there will be a higher number of defects in a piece of DOM tubing (due to the welding process) than a piece of truely seamless tubing. Also, machining the surface off of DOM can expose weld defects that were smoothed over and hidden during drawing/forming.
Howeverer, as I said previously, seamless tubing (as well as bar stock) is never going to be free of voids/inclusions either. Barrels made from them need to be inspected just as carefully as other. Any worry about DOM should be equally applied to
any steel barrel as far as I'm concerned because while surface defects (including
machining defects, BTW) will help start the process,
corrosion alone is going to cause more (and larger) voids than anything else.
Clean and preserve your cannon's bore like you would on any other BP gun and visually inspect it for corrosion frequently. There is no need for any type of special NDT to detect corrosion. If your barrel is of suspect material, by all means have it evaluated at a lab (if it's worth the cost). If not, cut it in half or weld the bore shut and put it out by the flagpole.
I would be very surprised to hear that any cannon barrel failure was found (by careful, scientific evaluation) to be due
exclusively to the use of correctly manufactured DOM tubing. I suspect that those who have examined cannon barrel failures where DOM has been used correctly (if there even are any such failures/examinations) have simply speculated that it may have been the main factor.
If one is concerned about the safety of a liner for a new cannon, why not purchase material that has been evaluated, honed and chromed? It's available if you want to pay for it...
http://www.scotindustries.com/index.php