S7 is sometimes called shock steel. SR77 is S7 after the Busse heat treat. It is not a special formulation. SR101 is 52100 after the Busse heat treat. Spring steel is like 5160 and used a lot in choppers. Justin Gingrich of Ranger knives uses it, as does Ontario, who now manufacture the Ranger line. It's pretty tough stuff. INFI is a totally unique steel made exclusively for Busse Combat. Busse has used many steels in the past, including ATS-34.