You haven't found any because there aren't any. FN said they were to going to release them this year and didn't do it.
You can buy a new factory 16" barrel and gas block and have it cut. They are close to $1000 IIRC
Hi-Desert Dog's quality control is hit or miss from what I have read. I won't be sending my thousand dollar barrel to someone who can't get it right every time.
Will work for Q3131