A way of asking an equivalent question might help it make sense. You could have asked "at what pressure would a 1000 cc bottle filled to 300 bar get to the same pressure as a 500cc airgun starting at 150 bar?". The question is a bit odd because the bottle is small. But regardless of the size when you take air out of the supply bottle it's pressure starts to fall. If the supply is pretty close to the recipient size then you may not reach your target pressure. The supply is only 50 bar higher than the target and you are trying to raise the gun 150 bar.
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