For what it is worth, DOM tubing is commonly called "seamless" tubing although it does have a seam running the length of it.
As those of you who followed the links or found other information on the web know it is actually welded tubing.
Following the weld, the tubing is forced (drawn) over and thru dies which smooth the surfaces.
The reason for putting the welded tubing thru the drawing process is to smooth out the weld joint and to size the tubing so it conforms to the surface requirements of various tube specs like AMS5560, MIL-T-8504, MIL-T-8606 etc.
This drawing process by the way will cover or hide faults and inclusions present in the weld joint by smearing adjacent metal over the potential problem.
DOM tubing has no testing done to determine if a weld joint flaw exists.
This drawing process is important if the tubing is going to be used with standard tube fittings which require a smooth, uniform surface to provide sealing. This is especially important with compression fittings.
Drawing tubing over and thru dies does improve the mechanical strength of the material by work hardening it. This is most noticeable with the non-heat treatable austenitic stainless steels but some degree of improvement in the low carbon steels is also seen.
Speaking of steels the most common material for DOM tubing is SAE 1020 a low carbon steel often used for "angle iron" and flat stock. Stronger steels with more carbon in them are available as are various alloy steels like "Chrome Moly".
(The potential problem with these is they all look alike. The buyer must obtain the Certification Sheets along with the tubing in order to be certain they have the material they need.)
Without this drawing process there is virtually no difference between a 1018 DOM tubing and plain steel water pipe if one ignores the galvanizing that water pipes have.
My reason for mentioning water pipes is because they typify the low pressure applications most DOM tubing is designed for.
Yes, some DOM tubing is designed to be used in systems with pressures up to 4,000 PSI and higher but if one reads the specs they will find that these high pressures are static pressures, reached gradually by, for instance, turning on a valve.
They are not the same kind of pressures as the explosive pressures produced by the explosion of black powder.
I should mention that there is REAL seamless tubing which is made by forcing hot steel (or other metals) thru dies.
This real seamless tubing has no welds so it is fully as strong as if it were machined out of a piece of bar stock.
It is used in critical applications where failures due to pressure could be catastrophic. Think nuclear reactors and submarines.
This REAL seamless tubing is usually made from high strength alloy steels, stainless steels and exotics like Inconel625, Inconel718, Waspaloy, Titanium etc.
Because of the critical nature of its uses it undergoes extensive testing ranging from ultrasonic to X-Ray.
Our problem as gun builders with the real seamless tubing is it is almost impossible to find in small amounts and it is Extremely Expensive.
Some have mentioned that the old rifle/smoothbore barrels were all welded and asked what is the difference?
Without making this post much longer than it is I will mention that the old barrels were made from wrought iron which welds differently.
The weld joint was a "scarfed" weld joint with more surface area being fused together rather than a butt weld with minimal surface areas being joined.
Not last, but definitely not least is the fact that ALL of those old barrels required proof testing and many of them, even after undergoing and passing their proof test ended up exploding and maiming or killing the guns owner.
