Fusion welding is a process that can be used for assembly of certain components of a plating barrel.  It is based on the pricipal that two pieces of plastic evenly heated to a given temperature ,  then held together under pressure , will fuse to create a hermetic bond nearly equal in strength to the original material , leaving no gaps or noticable seams.
  A company known as Wegener Corp , based in Germany , has pioneered this technology ,with machinery designed to fusion weld flat sheet material ,generally used in tank fabrication. It is an industry accepted fact that fusion [butt] welds done on a Wegener machine are hermetically correct , and 100% bonded.
  Unfortunately , Wegener has not [as far as I know] produced a machine that can fusion weld plating barrels.  The Wegener machine can be modified to fusion weld 5 panel sections into the hex form  , but since the hex form can be bent from a singular piece of material , fusion welding is not required to manufacture the hex form. 
  Attaching the hex form to the head plates is another issue , there are a few companies that advertise this joint as fusion welded. Since I don't believe the Wegener can be modified to this extent , I'd guess these are being done on custom built machinery [ not very likely] or by hand [most likely] using some custom made tools for the job.
  We are currently experimenting with fusion welding of the barrel head , and have proven it is possible to make a good fusion joint here , but it is equally possible to make a good looking joint with little to no structural value. Unfortunately , without the proper equipment , fusion welding becomes a dice toss , one that B&P is not  willing to pass on to our customers until the technology has been developed to eliminate the possibility of failure.