It could be insufficient flow restriction through the reg. If the reg opens due to pressure drop as the firing valve opens and flows air into the firing valve chamber during the open duration of the firing valve then the air res pressure will be contributing to the muzzle energy. The reg could actually be set correctly and sealing at a consistent pressure, if so restricting the flow through the reg to a slow trickle could solve your problem. The Valve chamber will still refill in a second or two after each shot.
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