Yea a little wiggle room never hurts!Be on the safe side and make the dowel 18 1/2”.
What I was told by a dealer is that they have an 18" dowel that they drop down the bore. That's why he always cut the barrel off at 18-1/4 inches.I'm more safe than that. I measure 18 inchesmfrom the front of the receiver. Call me chicken but some folks government abilities may not include proper tape measuring. Ha ha.
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