Having worked at many paintball fields, it is easy to do.
Co2 is a "self regulating gas", meaning the liquid in the tank adjusts to keep the pressure equalized to around 800psi, regardless of how much liquid is in the tank. So a full tank is 800psi, and a near empty tank is 800psi.
The only catch is the tank needs at least 25% "headspace" in order to properly equalize, or the pressure skyrockets up to 2000-4000psi.
So that 5lb tank could actually hold 7lbs if the headspace was accidentally filled, which is possible if the filler was not paying attention.
Vents are usually rated 1800, or 3000 (depending on application, and tank material). If the vent blew, it can only mean the tank was overfilled (or defective).
nick