In a HPA system the bottle is usually filled to 3000 psi or 4500 psi by a scuba tank, a dive shop, or a pcp pump for smaller bottles.
The high pressure air is then regulated down to a lower pressure typically 800-1200 psi by a regulator on the bottle. So even though the bottle pressure decreases as you shoot ,the regulated pressure going into your gun is pretty consistent up until the bottle pressure gets near the regulated pressure. For me it was a great repalcement for co2 since it is not affected by cold temperatures or rapid firing as is co2. I use it now on my RWS 850 and AR2079 and really like it and will probably never use co2 again. I have various regulators with output pressures from 750 to 1200 PSI . They are fixed regulators and can only be adjusted by changing the internal springs or shimming.
Jay