The regulator is the piece of the system that guarantees that even though your CO2 tank might be at 500 or 1000 or 1500 psi, only somewhere between 0-30 psi will be allowed on the keg side. You can dial it in to the exact number you want (for example, I generally use 12-14 psi in most of my ales held at 45-55° F.)
Opening a simple valve from a fully pressurized bulk CO2 tank directly into your keg would blow you and your keg to smithereens (if it weren't for the pressure relief valve built into the keg.) A paintball canister might not have enough gas in it to do that, but you still need to have some sort of precision in the amount of pressure on the keg side of the system or you'll end up with beer that's overcarbed, undercarbed, flows too fast, or doesn't flow at all. You can't just guesstimate.
On a garden hose, you can use the hose bib as a regulator for the water thats flowing through it, but when it comes to extremely high pressure gases, it's a different story altogether. You need an actual regulator--no real way around it.
On tap:Witbier, American Stout, Imperial Stout, Root Beer
Bottled:"Massacreation" Chinook Barleywine, Belgian Golden Strong Ale, English IPA, Cabernet Sauvignon
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