You want to use it for precision? Say what...
My opinion is, the FAST stock is not conducive to precision work. We'll leave all the other XCR accuracy jokes out of it.
Look at the lock up on other folding chassis or stocks that are built for precision. My AICS is 100% solid, without seeing the hinge you'd never know it folds. RA hasn't even come close to achieving the same. Which is fine because the rifle wasn't built for precision either.
This is all just my own opinion based on my own experience.
You'll do well swapping to a PRS or the like.
Edit to add:
I found my L to be a different rifle after swapping to Dlask/Magpul. I've got a CTR w/riser that I've custom mounted. The riser is canted slightly to the right. When I'm doing my mall ninjary, that stock is part of my face. I never had that with the FAST, it was more like what you're going through where I was getting slapped.
Good to know. Yea, I know people joke about XCR accuracy but there is proof out there they are capable of sub-moa groupings (especially the newest ones). Most people just use a 4x scope and poor shooting skills and say "this is only capable of..." so I honestly never believe peoples opinions on accuracy capabilities unless the test was done by a person with precision shooting skills. We will see though. I am trying to combine a hunting rifle, tactical style rifle, and precision rifle into one without spending 5k. Only time at the range will tell