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I've recently acquired an old (pre 1877 judging by the proof marks) Belgian single shot 8 bore muzzleloader. The barrel measures true 8 bore (21.2mm - 0.830 inch), but the original calibre is listed as 20.4 mm ( 0.803 inch). So I assume that at some point someone honed, or bored the barrel out.

I'm planning to "proof test" it with a load only slightly exceeding the service load for 8 bore rifle, but before I do that I'm trying to establish if there is any possibility the barrel may be too weak. I don't want to blow the gun up. If it is too weak I would rather sleeve it to a smaller calibre.

The outside dimension of the barrel near the breech is 1.47 inch.The barrel is octagon transitioning to round. Using few available calculators online they give approximately from 13500 psi to 32000 psi as maximum pressure a tube with this barrel's wall thickness can survive depending on steel type. I used numbers for historic steel strengths I found online.

So now the question is what kind of pressures should I expect with various black powder 8 bore loads. Ideally I would like to shoot few 820 grain round balls, and 1 15/16ths ounce of shot at 1200-1300 fps.

Does anyone know design parameters of 8 bore barrels? Specially design pressures, barrel thickness used in modern replicas etc? I found various charts online, but they all go up to 10 bore only.

Also, I found out steels available in mid 19th century had ultimate strengths from 220MPa or 32000psi (for lowest grade) to 530MPa or 76870 psi (for best grade). Does anyone know what kind of steel was the most likely non-laminated barrel steel in Liege at the time?

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