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Wikipedia has a good article on measuring backthrust and lists backthrust for some common cartridges- Google it,

Carpetsahib is correct in that it is the internal head area (on which the powder exerts the pressure) that is used in the calculation but you can also use the outside case head diameter in the calculation which then provides a safety margin for those loads that can give higher peak pressures.

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I agree that the outside head diameter can be used to provide a safety factor. I will point out that the adhesion of the body of the case to the chamber wall can also be taken into consideration. If you do so, it will provide an additional safety factor.

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