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 According to Mine Safety Appliances, a major manufacturer of scuba tanks, the highest internal temperature that is produced by rapidly filling a tank is about 116° F. That is from 0 to 4500 PSI in 45 seconds or a rate of 100 PSI per second. This is starting at 75° F using standard breathing air. The size and composition of the tank makes no significant difference in the amount of heat produced, it's safe to assume this applies to PCP tubes as well. This fill rate is not considered harmful to the tank. Insurers of scuba and paintball establishments do not consider it unsafe. Rapid filling of a tank or gun will not cause it to explode as long as it is properly maintained, the safe operating pressure is not exceeded and an unsafe substance is not present internally.

 Some older tanks are made of 6351 aluminum. It develops stress cracks when pressurized for long periods of time. This caused some of those tanks to explode. Use of that alloy for high-pressure tanks was discontinued by 1988 in the US. Most are made from 6061 aluminum now. There have only been a few tank explosions over the past few decades. The causes are not well documented. It's suspected those tanks were out of certification, corroded or critically overfilled. Another possible cause is a flammable or combustible substance, like oil or cleaning solvent, was present internally causing rapid compression detonation.

 Airguns are designed to withstand 3 or more times the pressure they will contain. As far as professionally built guns are concerned, there have been no documented reports of one exploding due to a design flaw or material failure. Outside of a couple suspected hoaxes, there was a highly publicized incident where one man was injured. He was taking his PCP gun apart with nearly 2000 PSI in the tube. He used one valve screw, instead of the three required, when he customized the gun. He didn't realize the screw had failed. The valve was only held in place by the front trigger frame screw which it had caught on. The transfer port moved out of alignment when the valve screw failed, he was taking the gun apart to investigate why it was shooting poorly. When he started to remove that front trigger frame screw, the valve shot out the back of the tube. The valve penetrated almost completely through his thigh. He publicly blamed this incident on the HiPAC unit he was using. He had modified the HiPAC so it pressurized the tube. It was originally designed to isolate the tube from being pressurized. The modifications he made to the valve and HiPAC would have required three screws to safely secure the valve. 
Airguns in general are very safe. Customizing one is safe if you know what you're doing.