Pretty sure military barrels are chrome lined to help extend barrel life when used at high rates of fire. The only full auto M16 I have experience with was inRVN and had at least 400 rounds an hour fired on occasion with out showing any signs of an effect on accuracy,carried it for a year and it was still doing fine when turned in. Target shooters claim chrome is not as accurate but I don't think the difference matters unless you are shooting paper for small groups. I have a clone that has a M 16A1 upper that has excellent accuracy and it was used but I don't know how much.

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