Have been some made from Al., and it has the strength (once anodized) for what you want. Were some made of brass as well, but it might be a little too "blinky" for many of us. Could square it off a bit and make full sized centerfire rings work.
Larger foot print to the gas tube would be a good thing for stability, and as you intend to free float the barrel, will need a nice solid breech/gas tube fit.
Know I've read about threaded barrels, but haven't tried one. Get the fit right, and it may not be needed. If you do, work on a detacable loading section that can be removed in order to get at the issue breech screw (which is under that loading extension of the barrel).
The transfer port has to extend down into hard contact with the valve or gas will leak between the valve head and the gas tube, squirting along the inside of the gas tube towards striker. A thick enough o-ring should work, so long as it's firmly compressed by the breech and the valve head (just passing though the gas tube).
Could add a "strangle screw" to adjust the gas flow though the transfer port in the riser breech (a transfer port limter)... would be a fast way to adjust from high power to low power without having to change spring tensions.
