There has been a lot of talk about DOM and HREW tubing among off-roading circles and I'll try to answer a few questions that you may have about the differences.
DOM is a process and means "drawn over mandrel". Typically you use DOM when you require precise thicknesses in the ID, OD or wall thickness of a tube. It is not seamless but is made from sheet steel, rolled and electric resistance welded and finally, "DOM'ed". DOM is a cold process. Most typically it is made from low to medium carbon steel.
HREW is cheaper than DOM, but isn't as nice. HREW has a seam weld, as does DOM, but DOM is taken a few steps further to counteract the welding and cold working abnormalities for a straighter stronger piece of tubing. But as far as strength goes for bumpers and sliders, HREW will sufficiently do the job except for applications that require extreme strength.
If I were building a purely competition rock buggy where I would be getting into areas with large drop-offs, I would probably use DOM, but for the kind of off-roading that most people do and even for the occasional rock competition, I would probably go with the HREW.
In summary, DOM is a stronger tubing material than HREW, but costs about twice as much per foot. HREW is more than sufficient for most off-road applications.
