There are a few ways to do this.
Simplest is probably to turn this from a piece of solid round bar on a metal lathe.
If it needs to be hollow, you could use a thick wall pipe for a starting piece.

I dont know where you are in the world, but in most industrialized countries, thick walled tubing or pipe is available, as it is used to make hydraulic cylinders for earthmoving equipment and similar stuff.

If it MUST be made from sheet metal, you can make it up from two halves, each 2.4 M long, which are bent or rolled.
You can make a series of bends on a press brake, and make two half cones, and then weld them together.

Yet another way to do this is to forge a tapered pipe. As a blacksmith, I forge pipe from time to time, and tapers are quite possible. This is not very precise, so I would usually clean this up on a metal lathe at the very end.

Or- you could cast the part in one piece. You dont mention what metal you are using, but aluminum or bronze could be sand cast into a shape like this.
Again, it would probably require using a lathe to finalize the shape.

The simplest, least tool intensive way would be to find a series of pipes, each a bit smaller in diameter than the last, and weld them together into a stepped cylinder, and then grind, grind, grind. If you get gaps, you add some welded metal, then grind some more. This will work, and requires only a stick welder and a small electric hand grinder, and a lot of time.

I'm a blacksmith, so I'd opt for the forging solution. Take a piece of pipe that is 3 to 3.5cm by about 2.4m long and forge it down to a taper. A "V" block having a 115 degree included angle that is also slightly ramped (about 1 or 2 degrees) will make it much easier to keep the taper uniform. Since I have a power hammer it would only be an hour or so work to forge the taper and another hour to true it up on the lathe or even with a hand grinder using the pipe threader to rotate during grinding. It helps, when doing this, to start with seamless tubing, bu tit does work using welded seam tubing. The particular metal doesn't matter too much; it works with steel, aluminum, bronze, copper and others. I've used all of those with complete success.

If you need to do this using thin sheet metal, one way is to start with the appropriately shaped piece of flat sheet metal and then start the curve in a V block with a narrow hammer and then pull it through a drawing die to bring it together at the seam. You would need a series of progressively sized dies, since this is a tapered piece rather than a straight tube. This will result in a series of stepped diameters, the number being equal to the number of dies you draw it through. After drawing and welding/soldering the seam, it could be quickly trued to a taper over a tapered mandrel.

Rich

If you could use a faceted cone instead of a round one you could easily have the two halves formed in a press brake and weld them together or even cut each trapazoidal piece and weld it all together, then grind the welds smooth. Should be able to have it done at any metal shop with a 10 foot(3 meter) brake which shouldn't be too hard to find.

Jake Balcom
Mettle Design
Lincoln, NE