DOM means drawn over mandrel which should give a rounder and smoother inside than plain tubing. However it is hard to see how you could make seamless tubing without some kind of mandrel.
If
this description from the
Steel Tube Institute is accurate, DOM tubing is NOT seamless. A google search on "seamless DOM tubing" brings up lots of people claiming to use it but I haven't found anyone claiming to produce it.
Here is a
Timken Steel Seamless Steel Tubing Overview with lots of interesting info.
