I ended up going back to the carbine stock. I really like this prism sight, but eye relief was a little too long on the rifle stock, and only got worse with heavier clothing.
I followed this excellent advice,
The PSA FN barrels are gassed to run worst case- weak 223 with an H2 buffer.
You'll want an H2 (H3 might be worth a try), and ensure you have the SOCOM bolt upgrade kit for the extractor to use 5.56mm pressure Ammo.
New extractor including new spring/plunger/o-ring, and an H3 buffer. Runs great now with an authoritative 'thu-thunk' as that H3 buffer drives the bolt home. Brass shows barely visible extractor claw marking.
I'm done.