Quote:
Originally Posted by alanint
The stripped upper receiver forms a "tuning fork" that when gently tapped on a non-marring surface, will ring. Repaired, re welded or messed with upper receivers are not supposed to "ring".
This is more lore than science, but some people do it.
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I when I learned of this test on Lugers, I had to chuckle. It reminded me of a similar and valid test performed on forged crankshafts for performance engines. If they don't ring when tapped, look for a hidden hairline crack.
