docEE, A good place to see most of the factory pressures for NE rounds is in Graeme Wright's book " SHOOTING THE BRITISH DOUBLE RIFLE". These are reprints of the old ammo makers. I doubt you will find any of the old true NE rounds made to be used in doubles that will generate 40,000 psi chamber pressure. The highest I find is the 450NE 3.25" 17 Emperial tons X 2240 (Emp long ton) = 38080 psi chamber pressure.

The 470 is listed at 14.0 E-tons x 2240= 31360 psi. There are some cartridges that are chambered in double rifle that generate more pressure, but those are generally small diameter rounds like 30-06, and 7x57R that generate around 40,000 psi, but they have plenty of steel wrapped around the chambers, and are not a problem because of pressure. Rounds like the 375 H&H, belted rimless, and 416 Rigby that cause the problems, with a combination of pressure, and the RIMLESS configuration.

The S/S double rifle is basicly a 30,000, to 35,000 psi system, that works best with a flanged cartridge, of low chamber pressure! As has been proven by the last 100 years of use, nothing more is needed, no matter what the gun rags say!