What makes the Czech ammo corrosive is the salts they use in the Berdan primer in the cartridge. Mod.43 just refers to the calibre, just as you can have all different kinds of M43 loads. You would see that quite quickly if you didn't clean your rifle after shooting Czech or Hungarian surplus, the rust sometimes only takes 48hrs to develop. (Had a buddy ruin an SKS this way). The reason the Commies used corrosive primers was that they felt that a corrosive primer had a more positive ignition (and less chance for misfires) in a cold climate. And in the 1970's Soviet Union, they were probably right. Anyways, as a guy who doesn't want to see you lose a great rifle to rust, I suggest you look at a few methods to clean your gun immediately after shooting it with the Windex treatment, or boiling water (regular cleaning won't dissolve the salts) or you can be a lazy bugger like me, and just shoot non corrosive, like Norinco, which is dirty, but won't require all the cleaning hassle.