I have some of the Yellow box stuff gave about $5 a box for it it just plain old mild steel core,some Chinese ammo factories made steel core some made lead core,all the bullet jackets on Chinese ammo will be copper coated steel so the magnet test is a moot point it will stick to all of it.
Be careful of you backstop if you shoot the steel core ammo,it tend to separate and ricochet off hard targets pretty bad.