If you really want it to use a gas instead of a liquid or gel there are two ideas that come to mind:
The first is a vortex cannon. Basically like blowing a smoke ring, only at a much higher velocity. https://www.youtube.com/watch?v=BuH-hWrjZmw has some cool demonstration shots. The kind of accuracy you're wanting would be a bit of a stretch, but possibly doable for a creature that was really good at judging the air currents on the path to the target. It would, of course, have to blow a lot of fire rings at a rapid rate to melt steel, but that's also potentially possible. Much easier with a liquid though as that would transfer the heat much better. Steel has to get really hot to melt.
The other possibility is a bit more speculative. I remember reading about ten years ago about some research on beams of ultrasound, specifically that sufficiently powerful beams of high-frequency sound in air decay in a pattern which causes the circumfrence of the beam's distortions of the air to create a low pressure area in the interior of the beam that keeps it much more coherent than would otherwise be expected for a directional sound impulse moving through the air. The researchers were hoping to be able to use the phenomenon to allow for "surround sound" systems using only one set of speakers and encapsulating the rear sound in an ultrasonic conduit that would be disrupted upon hitting the back wall making the sound seem to come from behind the audience.
This is quite speculative for a breath weapon because I don't know if the low pressure zone created would be sufficient to contain a propagating gas, but if it could be made so then it could be used to deliver your dragon's breath weapon with the kind of accuracy you are describing, and the ultrasound itself would add a not-insignificant amount of energy to the mix as well. I suspect, sadly, that producing a sustained beam of ultrasound at the required power levels would be beyond most biological or, indeed, mechanical systems currently in existence. But if you don't mind a little bit of magic or hand-wavium it would be workable for a story. And if your world includes magic along with the dragons it would allow some innovative uses of any "silence" type spells.