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[SigP510]

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[ar15barrels]

is there something else going on with this barrel that makes it "special."

The barrel extension is 1" diameter and will fit into standard aluminum receivers.
The reason that the barrel nut threads are larger on the plastic receivers is that some of the threads were breaking off the front of the receivers when they were using standard size barrel nut threads.

[SigP510]

Great, thank you Randall !