What is fill pressure and why does it vary by gun?

Fill pressure is the maximum air pressure the manufacturer specifies for the onboard reservoir, and it varies because reservoirs are engineered to specific working pressures.

Common values are 200 bar (2,900 PSI) and 300 bar (4,350 PSI). Filling above the marked maximum stresses the reservoir, can damage the regulator or burst disk, and on unregulated guns can actually reduce velocity by causing valve lock. Always fill to the manufacturer's number and no higher.