Purchased a copy of Brown's book "Building double rifles on shotgun actions" yesterday.
Interesting stuff and some good info included.
I must say though that his calculations regarding pressure on the standing breech are a little strange.
I'm no mathematicion nor ballistician but something seems a tad wrong to me.

Page 19 for anyone else who has the book.

One minute he is using the case head AREA in his calculations then he changes to using case head DIAMETER!!!

What the??!! Surely this is a mistake?

Also he states that the 16 bore shotgun cartridge had a max pressure of 12,000 PSI but claims his 16 bore action has been proved at 32,000+ PSI!
I thought most proofs ran at around 30-50% above max pressure not 265%!!
Wow, were any shotgun actions ever subjected to such high proof pressures and would the barrels survive?