Mr. Martin is correct regarding standard steel loads; limited to 400 m/sec or about 1,312 fps
This document (which may be old) specifies choke < .5mm = .020" and max. velocity < 430m/sec = about 1400 fps for High Performance Steel
proof, for shot 4mm or >https://www.cip-bobp.org/homologation/uploads/ciptexts/decision-xxxii-45-0-en.pdf
This is from 2017 https://www.shotgunworld.com/bbs/viewtopic.php?f=126&t=465245
This is dated 2020https://www.gwct.org.uk/media/1094678/GTA_factsheet_shootingnonlead_ver102.pdf
"‘Standard steel’ cartridges have been designed by manufacturers in association with proof authorities1that can be fired through any nitro-proved gun.They must have a cup wad to protect the barrel; they have a maximum shot size of 4
; and they have to conform to the normal pressure limits of nitro proved guns.
Trials in 1991 using standard steel cartridges with light loads (24 grams) demonstrated that even light walled game guns of contemporary manufacture with 3/4 chokes showed no damage after firing 1000 rounds. Standard steel loads can be fired safely through light walled guns but there is a risk that in some circumstances a slight bulging at the choke neck can occur. The likelihood of such bulging is increased by heavy loads, large diameter shot and steep, tight chokes."
"High-performance steel cartridges have limits on what should be used. Shot sizes exceeding 4mm (BB and larger) should only be fired through a choke less than 0.5mm (half choke)."