His math calculations are WAY wrong. Take a look at page 19 of the first edition of his book.
When calculating the strength of his 16g action he makes serious mistakes.

First he uses a grossly incorrect proof pressure of 32,000 psi. It has been claimed this figure was just a "typo" but it is not, because he actually uses that 32,000psi in his calculations.

His second big mistake, is that to calculate how much the force against the breech will be for the (incorrect) 32,000psi proof load, he multiplies that figure by .810". This is TOTALLY wrong.
.810" is case head DIAMETER on the 16g.
He should be using case head AREA, NOT DIAMETER, which will give a vastly different answer.

So, he has the proof pressure way wrong in his calculation, then he gets the math totally wrong when calculating the pressure against the breech.

The bottom line is that he wrongly calculates that his 16g action was proofed to withstand a massive 25,920 pounds against the standing breech.
This figure is WAY WAY wrong because he has used the wrong proof pressure and then also used the wrong case head area.

The calculations should have been done at 12,000psi for the 16g, multiplied by 1.3 to 1.5 for the proof pressure, multiplied by the CASE HEAD AREA.

12,000psi X 1.5 = 18,000psi

Case head area is radius squared X 3.14.
So radius of .405" X .405" X 3.14 = .515 square inches.

Multiply proof pressure of 18,000psi by .515 sq inches = 9270 pounds pressure against the standing breech.

His calculation was that the action was proofed to withstand 25,920 pounds against the standing breech, when in fact it would really have been proofed at less than 10,000.
That is a HUGE difference , and a dangerous one.

Again I stress that I mean no disrespect to Ellis Brown whatsoever.
I am merely bringing this to light so that people who have read the book don't get the wrong idea regarding the strength of some shotgun actions.