So what thickness are you saying is your minimum, JohnfromUK?
JR
There is no specific figure as it depends on how good the material is (in the early days often how free from flaws it was), and where the measurement is taken. Walls can really be very thin near the muzzles and won't burst, but are very easily dented.
I agree that 25 thou and over is a fair "guide' to a near guarantee of safety
but that is only one aspect, and 'proof' is a better test and will allow many that would fail the 25 thou test to remain in service.
Proof would also pick up hidden flaws and possible action weakness.
