He was calculating breech thrust of the load used to proof the donor action, and the formula he had been using up to that point was correct, that being cartridge chamber pressure multiplied by the AREA of the cartridge case head.

He claimed that the donor 16 gauge SxS was made in East Germany after 1950, and that it was proved at 32,000+psi!!

32,000+psi is an extremely high proof pressure for the 16 gauge, which would have a cartridge service max pressure of no higher than 12,000 psi.
32000+psi would be the service max pressure of 12,000 psi x 270%!

On top of this came the error.

Instead of multiplying the 32,000 psi by the AREA of the case head, he multiplied it by the case head DIAMETER! (.810")
This gave a totally incorrect figure of 25920 pounds force against the breech!

To give you an idea of how high 25,920 pounds of breech thrust is, it's very close to the breech thrust of a 140% proof load for The 460 Weatherby Magnum!

If he'd correctly used the case head AREA (.515 sq inch) instead of the DIAMETER, the breech thrust figure would have been approx 16,480 pounds NOT 25,920!

But I'm still suspect of the claimed 32,000+psi proof load, and have to wonder if it is correct. Sounds way too high to me. If so, the true breech thrust figure would be considerably lower again.

The bottom line is that the errors that were made, suggested that the particular 16g donor action was much stronger than it really was.