I am not a FX technician... Don’t work for FX either... But, I have modified, broken, and repaired my own FX PCP’s for 5 years now, as long as I can order the part(s) I always got the airgun back going. From leaks, to broken parts, to stripped screws, weird random losses of power output, and problems caused by my own tinkering.
Shooting your FX PCP below the set regulator pressure a bunch could possibly but not guaranteed to cause the following problems.
1. could break your valve pin
2. could break your valve seat
3. a broken valve pin with continued shooting could damage, scratch, mar-up / scar-up the inside of the PCP’s block where these parts are installed and need to be smooth to operate properly.
4. shooting your FX PCP below regulator pressure can cause your hammer weight & guide to over-travel, again causing nicks and scratches to these and connecting parts which need to be smooth in order for the gun to operate properly. Also, to produce consistency in shots.
5. regulator internal damage
6. broken springs
7. a leak
Advice... Try not to do it? May not cause immediate damage but if done continuously it can. Personally, owning my own carbon fiber fill tanks and a Omega Air Charger Compressor, when any of my airguns reach 5 to 10 bar near there regulator’s set pressure I go ahead and refill them. Pretty simple on my end to do.. No drama with FX either, life's good!