Proof firing

[scharfschutzen63]

I just happened to check. I have a no4 and a no5 enfield. Both are proofed 18.5 tons per square inch.
Enfield 7.62 barrels are proofed at 19 tons.
This is only 1000 psi more I am thinking. This is not much in relation to the overall pressure, but could still be beyond the failure point for a part that passed the 18.5 ton proofing.
19 tons per square inch is 38000 psi. In metric tons, just under 42000 psi
SAAMI Spec for 308 Win is 52000 cup.
I am thinking the units are different.
Can anyone shed some light on all of this?

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