Most surplus 7.62x54R ammo is steel jacketed, but not steel core. You don't see much of that anymore as it's not been importable for quite a while. One should assume that any surplus ammo is corrosive. CB
Most of the surplus light ball is steel core, although its mild steel not the ap stuff. You can look up the headstamps for information on
http://7.62x54r.net/MosinID/MosinAmmo.htm