What do you mean? Are you talking about contouring? In that case, you most likely would order the barrel already contoured to your specification, and then just fit them to the gun.
If you are talking about contouring, you could either do it with a tracer setup, or a CNC lathe - with CNC being the far better choice. A new Haas CNC toolroom lathe, model TL-1, has a work envelope of 16x30", and costs 23,000 BEFORE all the optional features. The TL-2 jumps the price to 27K, and the max length to 48". The first will do 98% of the barrels on civilian guns. The second will do 99.9999% of all civilian sporting guns, and they have CNC lathes all the way up to 35"x80".
If you're not talking about contouring, and just about fitting it(threading, chambering, etc), that's done in the headstock, and it doesn't matter how long it is, because it's supported at the other end by a "spider"(seen
here supporting the action end of a barreled action, probably for threading the muzzle). The issue with lathes is the headstock diameter - most anything 10" or smaller has a sub 1.375" diameter headstock thru hole. That generally means you can't do centerfire barrels as it won't fit through the head stock. CtoC length DOES NOT MATTER when using this method - for example, a Clausing 5914 has a 12x24" envelope, yet because you're doing the work in the headstock, you can thread 36" barrels... So long as they're less than 1.375" in diameter.
HTRN
