So, I've gone down a bit of a rabbit hole here. Someone on here may have been wondering the same question as me.
Will the FX-9 fit a HERA CQR butt stock?
It doesn't fit perfectly, unfortunately. However, it's just a matter of there being a gap between the castle nut and the start of the CQR stock.

My solution to this is I commissioned a friend that does 3D modelling to make a STL (3D printer file) for a part that will perfectly fit this gap.
Our thinking is the FX-9 + HERA CQR is such a niche combination, it's not worth putting the file up for sale, but to be 100% sure, I'm gauging interest to see if people would be interested in the final iteration once we have the dimensions dialed in.

Will the FX-9 fit a HERA CQR butt stock?
It doesn't fit perfectly, unfortunately. However, it's just a matter of there being a gap between the castle nut and the start of the CQR stock.

My solution to this is I commissioned a friend that does 3D modelling to make a STL (3D printer file) for a part that will perfectly fit this gap.
Our thinking is the FX-9 + HERA CQR is such a niche combination, it's not worth putting the file up for sale, but to be 100% sure, I'm gauging interest to see if people would be interested in the final iteration once we have the dimensions dialed in.