"The larget shot size makes them more destructive than the SG Bullets."
Slag bullets are used as ammunition for the shotgun in Dino Crisis.
They can be an option of ammunition for the Franchi PA3 or Franchi SPAS-12. Their larger shot size makes them more destructive than the SG Bullets. They are effective against all kinds of dinosaurs. However, they should be used sparingly as their supply is rather limited.
"Slag" could possibly be a mistranslation of "slug", a type of shotgun ammunition that consists of a single, solid projectile.