I have made muzzle loader barrels before, about 3 total. It's a HUGE amount of work to handforge a barrel, and I will never go through that again after the first one. The rest I did with swages on a trip hammer using a mild steel mandrel, which I cooled in water as I worked to keep it from welding to the barrel. My technique for rifling was not normal though and I did it on the spur of the moment. I basically ran a square cutter tool through the barrel, which cuts at each of the corners(you could use a hex tool for this), and dragged it through with wire and a crank. It cut 4 deep, straight grooves in the barrel. Then I cleaned it out with barrel ream made for shotguns, you know those flex hone ones... and then simply heated the barrel up and twisted it carefully with the round mandrel still inside. I straightened it out afterwards and with a bit of work after that it became a muzzle loader barrel. Much harder than the swords I've made, much much more work. One of these has since gone on a rifle for a friend I owned a favor two, and the other are still sitting in a box with the stocks I have not finished yet(I will someday though!!).
Now this above technique works GREAT for a muzzle loader, but for a modern rifle could use a more precision version of it. For the barrel forge two pieces of some allow steel(I imagine 5160 might work, or 6150(I think they make leaf springs out of this...) talk to a metalurgist and gunsmith for a better idea...) together around a mandrel and then tig weld the seam with a rod of the same material. Then forge it some more, run a more precision version of my ream idea through there, maybe one with six edges, and then clean and twist. The real part you'll have a hard time with is the heat treating. DO NOT DO THIS YOURSELF THE FIRST TIME!!!! You MUST either be a master metal worker, or a master gunsmith to do this without some consulting. It's extremely dangerous in a modern gun to not treat the barrel correctly. In my muzzle loader, it's not a big deal, but even in a small calibur like .223, the wrong treatment could blow your gun up in your face. You will also have to use a more careful firing technique with the high pressure gun than with a muzzleloader. You can't have any pitting or excessive fire scale. You will also have to finish the barrel up a bit after the treating, just running a hone through it if nothing else. In disclaimer I've never done high pressure barrels before, and I am just some guy on the internet, so if you proceed to try my ideas out, be sure that a master gunsmith keeps an eye on what you are doing. You can screw around with these ideas to your heart's content, but do not stick them in a gun that you, or more importantly, someone else, may use before a master gunsmith looks them over with a magnifier. It would also not hurt at all to have a rockwell tester available, and also to find a place to x-ray the finished barrels for you.
Ok, that out of the way... just buy a barrel...