Winchester Proof Steel is indeed Chrome Moly 4340 or 4130 and was introduced in 1931 for the Model 21 and reported to have a tensile strength of 115,000 - 120,000 psi with an elastic limit of 105,000 psi.
https://books.google.com/books?id=9S...YC&pg=PA57&lpg
The "Violent Proof" (VP) load was reported to be 7 1/2 (long) tons = about 24,000 psi by Burrard's conversion
Steel
target loads have a thick shot cup and I would certainly use them in a M21 with no more than an IM choke. Sadly more gun clubs are being forced to convert to non-tox only ie. Naperville (Ill.) Sportsman's Club and some in CA
Hybrid Mode