MCW
Somebody's talking crap here & it ain't the tree!
Hydro form them is the best, neatest way.
Most guys weld them, so they can make it a neater fit around the brake handle etc.
That's a very specialised job, guys like me are flat out changing a spark plug, let alone something so technical..... There is a couple of guys on the thread that are pretty handy in the calculated design of a particular pipe suited to whichever saw they choose to put it on!
Pfffft. You race guys always try to make tuned pipes sound so technical but I've been hammering successful race pipes out of old 20L oil drums since I was 10 years old.