SixBears;
Your figures are interesting & appear to be correct. I would however caution on one point. The 70K is as you stated tensile, or as sometimes referenced ultimate, thus the bursting point. The point which is really of most importance for a shotgun bbl is the yield or elastic limit. This is the point at which when the pressure falls away the bbl retains a swell/bulge rather than return to its original shape. This would most likely be in the 40-45K range for the quoted steel having a Brinell Hardness of only 140. I have data from Machinery's Handbook for a 1035 steel in soft condition @ BH 170 showing ultimate of 83K & yield of 51K. Thus the actual usable pressure will be less than indicated lest the bbl be bulged.
