I was considering the following:

12 ga. "rim" diameter - 0.886"
45-120 rim diameter - 0.608

pressure of 12 ga. 3-1/2" - 14,000 psi

12 ga. rim surface area - 0.618 sq in.
45-120 rim surface area - 0.291 sq in.

ratio of rim surface area = 2.124

12 ga pressure x surface area ratio
14,000 x 2.124 = 29763 psi

Do you consider 30,000 psi a good number... to low...to high?