I'm making my own airsoft gun and I need a pressure regulator for my HPA tank. It outputs 800 PSI which I need to bring down to about 130 PSI. Problem is, air pressure regulators that support 800 PSI cost around $120. I don't have the money for this, so I'm wondering if it'd be possible to use a ball valve that's almost completely closed to reduce the pressure. I found one that supports up to 1000 PSI on Amazon for around $7.
NO , a ball valve only will control flow. If you even allow a slight orifice the size of a human hair with time the chamber pressure would be at 800 psi.
I would look for a used scuba regulator they can handle up to 3000 psi and the first stage around 135psi , their actual output pressure can be adjusted a little. I mention this because I used an old scuba regulator to charge my potato guns. Other possible regulators C02 I think they are 1200psi with the output being adjustable, oxygen or acetylene regs, Try Craig’s list or eBay for used regs possibly from a welding shop call and ask. There are lots of options that will be much safer.
Do not use a valve you need some kind of regulator.