Seamless pipe does not exist. Pipe comes in a variety of wall thicknesses based on it's "schedule". Standard pipe is sch 40. Pipe sizes are mostly nominal, that is to say the sizes often mean little, for example 2" pipe is 2.375 od and wall thickness is 0.154, giving 2" pipe an id of 2.067. Wall thicknesses can vary by up to 12%! 14" pipe and larger reverts to exact size od, but from 14" to 42", the wall thicknesses are all .500". (Sch 40)
Tube on the other hand has sizes that actually make sense. 2" tube is exactly 2" od with dozens of wall thicknesses and materials available. Tube can be seamless or welded.
What is actally more usefull for cannon makers is known as "hollow bar". As the name would suggest this is round bar stock with a hole in the middle. Wide range of materials and dimentions, but be aware that the hole in the middle may not be concentric. This stuff is available mainly to save a ton of machining when making, well, cannons I guess...
Hope this helps.
John