For 3k to 120PSI reduction there are dedicated HPA products aimed at paintball/airgun market. I have one, works ok for low volume use. Also paintball gun regs will drop 4k to 800psi and you can go from there as an intermediate stage. Welding regs take 2-3kpsi to 15psi or so, so not good for your needs.
Keep in mind that if you use the HPA for breathing at all, you really should not connect it to a LP system to prevent contamination. Also keep track of what lubricants you use, particularly in the HPA side to avoid an O2 ignition as I mentioned in my first reply. Sure you can get away without these considerations, until you don't, rapidly.
PV = NRT is correct
Not so much, actually. It is sort of true in high school science classes, but it leaves out a lot of thermodynamics that happens in real world scenarios. In particular it doesn't apply to air compressors as it tells you how any two variables are related if the other variable is held constant. So you can do P vs V at constant T, or P vs T at constant V, for example. If you compress air then P, V and T are all changing at the same time and a different formula applies ( \$Pv^\gamma=\textrm{constant}\$ is the basic one).
This is it! This equation and compressors are not a static condition is why my thermal numbers are so much higher than others using just PV=nRT. You have to consider the
change in condition, not the final state. PS: The 840W required energy is the compressor input and not the air heat output, therefore you need to cool the heat from compressor inefficiency (35% minimum I'd guess; 15% from motor, 20% from seal friction/valves)
and the air compression heat.
er, nobody tests high pressure air systems to failure with high pressure air!!
If you need to carry out burst/failure tests, do it hydraulically like everyone else. Costs are small, safety is maximised. In the UK, i would be in breach of a large number of laws if i were destructive testing high pressure air systems in any domestic environment.
If you want an air gun, fair enough, just use the normal small capacity cylinders designed explcitly for this. Your local dive shop or air products place will fill that cylinder for you very cheaply!
Seconded, also hydraulic failure is less likely to send the broken bits to the extremes of your safety cage making post-failure analysis much more useful. Pneumatic failures often leave little intact to access, particularly in brittle plastics.
An interesting side note, often you will see HP hydraulic systems (oilfield) tested with HP N2 (in R&D, not in production/service) so as to ensure that an air tight seal will surely contain an oil/water fluid (due to the relative molecular/atomic sizes in play, high vacuum systems are often tested with helium). 15kpsi of N2 is fun stuff and these days I hear (I changed industries 5 years back) they're up to 30kpsi in the offshore safety equipment, no thank you.