i recall this was done for a while on NM rifle. i also recall on jouster.com. i think it was gus fisher saying that they stopped doing this because it makes the piston diameter slightly smaller and thus you get a bit more gas blowing past the piston. and for maximum accuracy they found the unpolished gave smaller groups.
now you'll have to be a master with the rifle. i know i'll never be that good. so i figure why waste the money on a polished part.
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