Are you sure there is one, as opposed to there simply being a sufficiently narrow tube/channel to achieve the same effect?
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The long and narrow tube will actually increase the pressure while the air restrictors spread the pressure out to dampen the speed.
You should be able to see all the way through the barrel when you remove the air restrictor right?
Yes, it depends on how you remove the air restrictor. I drill it out with a 3/8 drill bit., so I can usually see through to the plunger.