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The .450NE 3.25" at 44,000 psi chamber pressure shows approx 10,000 pounds force against the breech.

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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.

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Hi Carpetsahib,
That's a good question, and one I'm happy to answer.
I use the standard proceedure to calculate breech thrust, that being area of base in sq inches, times pressure in psi, but I never use the case ID, but prefer to use cartridge base outside diameter.
The reason for this is that in the event of a case head separation, the breech thrust needs to be calculated at base outside diameter, not inside.
Case separations do occur with reloads in DR's from time to time. This is not related to the cartridges being loaded to an excessive pressure, but rather that the cases have been reloaded one time too many.

Regarding the case gripping the chamber walls, as you say, the amount of grip, and the amount of diminished breech thrust is an unknown amount. That is one of the reasons I don't factor it in, because there is no way of knowing what to factor in.
Another reason I don't include it is, again, in the event of a case head separation, there will be basically no grip and therefore no diminished breech thrust.

When calculating breech thrust, with safety in mind, I prefer to do it as a worst case scenario.
This means (for me) using base outside diameter for the calculations.

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