N/A Regulator

Hello guys hope you are doing well.
I'm facing the problem regarding my regulator for Artemis P15. When i fully filled my gun to 240bars it gives me 740fps on 18 grain pellets and when the pressure decreases to 150bars it gives 850fps. Regulator set on 120bars. Don't know what is the problem. I changed the o rings but the problem still exist. Kindly tell me solutions. Thanks in Advance ☺️
 
Sounds like the regulator is operating in bypass mode. Meaning input pressure = output pressure. There was a similar topic a few weeks back so I'm going to paste in the content:

Your thinking is on the right track. If it is operating in bypass (output pressure = input pressure), the cause may be one of the following:
  1. The stack is squeezing flat before the piston can reach the valve seat.
  2. The piston is reaching the valve seat but the valve seat is damaged (e.g. badly worn) and therefore will not seal to halt the flow of air.
  3. The O-ring on the adjuster is bad, allowing air to bypass the valve seat.
Number 3 is pretty common so I would say look there first. But also inspect the condition of the plastic seal of the valve seat. If it is badly indented, the regulator likely will not work well after you’ve replaced the adjuster O-ring. Note that if indeed the adjuster O-ring was the culprit, the seat is likely to have experienced a brutal squeeze resulting from the overpressure condition, in which case it will need to be either replaced or wet sanded flat and smooth.

Link to the thread if you want to read through it:
 
It may be the bypass idea suggested already. That would seem to require a replacement regulator. Before you do that, how is the hammer spring set relative to maximum velocity for the regulator setting? I have 3 P35s and their regulators don't seem to like being far from the hammer spring setting that gives maximum velocity. But they don't do what you describe, they just give me a low first shot velocity before working normally. But if you are far from the point of maximum velocity I would reset the hammer spring and make sure the problem persists. I cannot get my velocity 5% under the maximum as is sometimes suggested with these guns. They don't work right if I try. Maybe 1 or 2% under.