Tubing question

I have to step down the fittings to the line lock. And to reduce the amounts of fittings I would have to use I'm using a prefab piece of tubing withe the flairs and fittings on it. I haven't been able to find a prefab piece of tubing with 12-20 fitting on one end and a 7/16-24 fitting on the other side in 3/16". Hope that makes sense.