In a nutshell that's kind of correct. However I don't think the pressure really build up so much as reach an equilibrium when the pellet gets moving. Initially almost all of the pressure drop is at the valves and orifices because the pressure on the other side of the mag valve is atmospheric pressure so the initial flow rate has almost all of the pressure drop right at the mag valve.
At some point pressure builds up and gets the pellet to pop past the hopup and start down the barrel.
The pressure drops I'm describing are not due to leaks so much as actual flow resistance. For instance if you crack open a valve on your BBQ tank a 115psi pressure drop occurs across the valve itself between the inside of the tank and the environment. Flow through the valve accelerates until all of the pressure drop occurs over the valve and you have a flow rate which drives a 115psi pressure drop over the valve resistance.
Imagine applying 30psi to one of a water hose to drive the water. At one end you've got 30psi. At the open end you've got 0psi (atmospheric) so you end up driving water as fast as 30psi can deliver over the entire flow resistance of the hose. No side leaks, just the flow resistance of the hose which delivers a certain flow rate of fluid for a given pressure drop.
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