I don't own a SFAR(never even handled one) but I do have a fair amount of experience with the common AR platform.
Here's what happens when you try to run a rifle gas on a 16" barrel: the 'dwell time' is often insufficient to achieve proper cycling. Dwell time is a function of bore diameter, gas port size, gas volume(of the cartridge used), and the distance the bullet moves beyond the gas port which what maintains gas pressure in the gas tube long enough to provide the force to cycle the bolt.
I've had excellent results using rifle length gas and a .223 18" barrel but this might not work with a different cartridge. Using a rifle gas with a 16" barrel cuts the dwell time to nearly nothing and function would be questionable IMHO. At the very least, the gas port would have to be larger to get more gas quicker--very tricky process.