Originally Posted by
lysander
The diameter of the bore in the upper receiver is specified to be 1.000" +.002"/-.000 for the first 1.110" (the threaded end) and 1.000" +.006/-.000 for the remainder, prior to anodizing. The anodizing is required to be .0008" to .0012" thick, reducing the bore to 0.9988", then a coat of solid film 0.0002" to 0.0003" thick is applied, bringing the final minimum diameter down to 0.9982". (The maximum diameter is 1.0008" for the first 1.11" and 1.0048" for the remainder.)
Those dimensional requirements have not changed since at least 1972.
By the way, the gauge dimension is 0.9981 +.0001" with a gauge length of 2.880". This is not only to ensure the diameter meets the minimum, but the bore straightness.
If the web below the ejection port is bowed inward, it indicates the order of machining of the upper was incorrect. The 1.000" bore should be bored as the first operation, when there is the maximum material, if the bore is machined after the ejection port and bottom slot, then the web is unsupported and will deflect outward, rather than being cut.
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