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Issue with my refilling from my 6.8l 4500psi CF tank

The other day I was out shooting my rifle. Which was the FX impact M3 and when my pressure got low, I hooked up my CF tank to refill the pressure at 190 bar and I slowly opened the valve to repressurize the bottle on the gun, but it would only pressurize to about 260 bar with the valve completely open it wouldn't go all the way to 300 bar. This happened twice to Me. The temperature outside was about 45degrees Fahrenheit, so it was quite cold. But I'm quite new to ant issues and resolving them with a CF tank. Has this happened to anyone else? Could it be the temperature outside was affecting airflow? Maybe the valve I have ? Which is the IORMAN one off of amazon with the red valve handle. Any help would be awesome!
Never trust an FX gauge