Running Vertical or 6-G welds is common for both FCAW and GMAW...
Vert open roots are common for GMAW short circuit.
GMAW with short circuit transfer can be run Vert up or Vert down depending on procedure requirements.
FCAW must be run with an upward progression with Vertical welds are required.
But here is the important part of the post... Vert parameters are much tighter than flat and horizontal. Amperage, voltage and stickout must be exactly correct and consistant.
Vert parameters with FCAW will be so tight in fact that you often have to change parameters to weld with two FCAW electrodes with the same AWS designation made by different manufacturers.
Here is a link to an excellent example of a manufacturer parameter sheet for FCAW
http://www.hobartbrothers.com/pdf/datasheets/Excel_Arc71.pdfNote that it gives parameter ranges for:
Different wire diameters
Different positions
Different shield gasses
For welding a continuous pass (180 degrees) on 6-G pipe with FCAW I would recommend the lower parameter Vert range given by the manufacturer. Doing this will allow consistant welding in all positions. Once you have the parameters down its only a matter of keeping the gun angles proper for the whole run.
Let us know how it comes out for you.