There are LOTS of threads regarding steel shot in BSS, and lots of...uh...bs
Miroku has not revealed the composition of the "Special Steel" barrels, but it is widely believed to be 4140 Chrome Moly.
And as observed the boxlock BSS is not a light-weight game gun.
Japan is not a member of CIP, but any Miroku sold in a CIP member nation must be proof tested to CIP standards, starting in 1969, though England did not become a signatory until 1980.
“Standard Proof” is for lead or steel - limited to no larger than 3.25 mm / U.S. No. 4s and max. fps 1,300.
As Stan observed, it is recommended to use choke no tighter than modified (unless using steel No. 7 target loads)
