Tim: I did some looking and couldn't find 2 1/2" steel loads offered today

Thank you Parabola
Are you saying that British Standard Proof is NOT for steel?

This is from the CIP site

12g 50mm, 60mm, 65mm, and 70mm “Standard Proof” lead or steel (limited to no larger than 3.25 mm / U.S. No. 4s and max. fps 1,300)
(Numbers are transducer BAR converted to PSI)
Maximum Average (SERVICE) Pressure 740 BAR = 10,733 psi; SAAMI is 11,500 psi
Maximum Statistical Individual Pressure 850 BAR = 12,328 psi
Mean PROOF Pressure 930 BAR = 13,489 psi

BTW: CIP allows much higher pressures for 3" loads than does SAAMI. 12g 3" is 11,500 psi and 3 1/2" is 14,000 psi
12g 3” & 3 1/2” “High performance/Superior Proof”
Maximum Average (Service) Pressure 1050 BAR = 15,229 psi
Maximum statistical individual pressure 1200 BAR = 17,405 psi
Magnum proof 1320 BAR = 19,145 psi

High Performance Steel regulations: the barrels should carry the High Performance Steel Fleur de Lys stamp and be marked “Steel Shot”

More
https://www.gwct.org.uk/media/1094678/GTA_factsheet_shootingnonlead_ver102.pdf
"‘Standard steel’ cartridges have been designed by manufacturers in association with proof authorities that can be fired through any nitro-proved gun."
"Increased velocity can also be achieved by changing the propellant and generating more pressure. Such cartridges are known as ‘high performance’ steel. They should only be fired in guns proved for steel. This is indicated by a ‘fleur-de-lis’ mark on the gun and the words STEEL SHOT."