All the tubing needs to be machined. It comes as round stock( a tube ). the only thing I would suggest you do is ask any gunmaker to actually measure the bore diameter of the stock he buys to make your barrel, or, if its made elsewhere for him, then ask him to measure the bore exactly, and give you the dimensions. The Steel manufacturer pump these pipes out in huge volumes, and they don't necessarily take the time to make sure how close the inside dimensions are for what they sell. There idea of tolerance is often a lot different than yours, or your gunmaker.
For instance, my gunmaker ordered my .20 gauge barrel stock from the same supplier that he bought his own barrel stock from. His barrel measures right on the money; mine is oversized. The result is that 20 gauge wads are too loose to seal properly, and I had to order 19 gauge wads to get a good load. I am talking a difference in velocity of more than 200 fps, and a much dirtier bore, as evidence of shooting an undersized wad.
The market for barrel making steel is very small, compared to other markets for steel. The producers just don't produce special steels for this purpose only. This is true for all steels used in rifle, pistol, and shotgun barrels. The larger companies have their own testing facilities, and reject steel stock that does not meet the specs. I have yet to here of a gun company who is large enough to be able to demand that the producers take back the out-of-spec steel barrel stock, and replace it at their own expense. The smaller barrel makers are stuck with selling the out-of-spec stock as either scrap, or find some buyer who will buy their small lot of steel at some better price. That is one reason that custom barrels cost a lot of money from the small makers. Oh, they really do a fine job in boring and cutting and polishing the barrels, so you do get a fine barrel. But part of their overhead is playing roulette with the steel producers, or jobbers, guessing what will actually be delivered, as opposed to what they ordered. Paying an off-the-premise lab to test the steels is also an added cost of overhead, that they have to pass on to customers.
In the industry, you will get a hot argument about whether a seamed, or seamless barrel is stronger. By the time a tube is machined to make it a barrel, it would be next to impossible to see yourself if the barrel is made of seamed or seamless steel tubing.
