What formula did you use to calculate the back thrust? My calculation is a bit less, namely 7542 lbs. of force. My calculation is thus: Area of inside of case (in^2) x Peak Pressure (lbs/in^2). Specifically, a .450 Nitro case has an inner diameter of appx. 0.462 in., with an area of .1676 in^2. operating at 45,000 psi. Also, this does not take into account any adhesion of the cartrdge case to the chamber walls: this will tend to diminish the total back thrust by an unknown amount.