It seems that there's a belief by many that all welds have porosity.
They do, to some extent.
It's impossible to remove all contaminants from the weld area that when heated, create gas bubbles in molten metal. Contamination can come from gunk on the surface of the steel, foreign matter in shielding gas, flux, slag, even inclusions in the base/filler material themselves. One can only hope to
reduce contamination (and thus porosity in the weld) to an acceptable level for the application.
Since the time I was in welding school in the early 70's and through my welding career, I learned that a good quality weld does not have porosity, micro or otherwise. If any welds have porosity, it is a bad weld.
We would have to define "a good (and bad) quality weld" for our purposes. It will be different than when I worked for a company that sealed radioactive source housings via electron beam welding in a vacuum chamber. Even those were sometimes rejected due to voids/inclusions detected with ultrasonic inspection.
A good weld is as uniform in grain structure as the surrounding base metal.
While I don't agree the above can be used as a blanket statement covering all types of welds, I believe it does apply to
most common DOM tubing. DOM is resistance-welded (no filler material), and in the low carbon varieties the weld is going to have the same grain structure (and tensile strength) as anywhere else on the tube. I believe an exception might be for one of the alloy varieties (4130 aircraft structural tubing being the most common) that has not been normalized.
As M&T noted in their original post, DOM tubing goes through multiple inspections to ensure the integrity of the entire tube, with emphasis on the welded joint since it's the probable weak link in its manufacture.
I personally believe, based on its attributes and uniform QA that makes it a commodity today, that DOM tubing should be considered to be seamless for the purpose of cannon barrel/liner construction.
If anyone can dispute that, please do so (with something more than anecdotal info).