Young Hang and Caliber gun

Why go past 250 bar? I read they Kalibrguns are rated 300bar but seals can and do blow when filled that high.

I don't quite get it I am afraid. The guage indicated 4000 psi which is 276 bars. Where did you get the reading of 250 bars ?

The last thing you need to worry about Yong Heng is it's filling speed especially when you are filling the gun directly. It consistently fills my Huben K1 from 150 to 300 bars in about 90 seconds. 

So you meant the pressure in the hose and the airtube of the gun failed to equalize or the gauge reading of the compressor did not agree with that of the gun ?

1) If not left in place long enough, the pressure in the fill hose and the pressure in the actual tube of the air gun may not have equalized. Fill it, leave everything in place for a few minutes, let everything cool, and you will get a more accurate reading. After that you will have a much better idea of where your actual pressure is.

2) Differences in the analog gauges of compressors and air guns are fairly commonplace, in my experience. 

If the problem is due to the excessively high filling speed of YH causing the pressure in the hose significantly higher than that in the air tube of the gun, a solution may be to add a gold filter. Besides removing moisture, the filter can also function as a small tank to buffer up the compressed air and transfer it to the gun ( slowly ) after the compressor is off.