Theory time!
On a cleaned and lubricated upper receiver the interior of the phosphate upper is kinda ragged, and not smooth if you look at it with high powered magnification.
Let's say it's possible for the FZ Carrier to drag on that surface...
However...
combining lubricant, with carbon creating a smoother, and slicker ''mud'' combined with the inherent lubricity of the Nickel Boron, and you're not dragging the carrier on the same ''ragged'' surface, your dragging it through an aluminium tube with a mud lubricant on it, that gives way to the hot ass piece of metal being pushed through said ''tube''.
tl;dr:
Upper receiver's micro jaggedness possibly caused by phosphate doesn't give way because it's a metal coating. Lube + Carbon =/= mud, which does give way, and lubricates the gun further.
Probably isn't true but it's a theory.
Also I'd like to note something:
You're gas port may be nearing .085-.090'' because of gas port erosion, if you've fired 8k rounds through it. If you're running a super large gas port already, then it's allowing even more hot gas through it which will further speed up the port erosion, and give you more gas... smaller ports take longer to erode because there is less room for hot gas to push into to begin with.
Hope what I said made sense...
We miss you, AC.
We miss you, ToddG.
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