There is no documented standard. The US does not have proof laws and never has. “Minimum and acceptable wall thicknesses is a topic that has been discussed at much length here and in nearly any other shotgun forum online. And it does have a lot to do with personal opinion and experience. Some searching of past discussions as well as review of British proof standards will likely enlighten you.
By the way. You shot the gun and it did not blow up. So that is a good thing. AND, if the gun has choke in it, which it likely does, the thickness at the muzzle means nothing. And it is no indication of the thickness farther back where it matters. You cannot measure wall thickness with a dial caliper. You need specialized gauges that cost at least a few hundred dollars.
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B. Dudley
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