copper over steel = bimetal. almost all steel cased x39 uses bimetal jackets
Any steel in the projectile can damage backstops, in theory. The bimetal is a lead core with a steel jacket and then a copper jacket over that. So it's essentially steel jacketed ammo.
Side cutters go thru the bullet like butter, i have that same ammo, there's no steel in the core but bi-metal jacket does stick to a magnet.Magnets don't stick to lead appreciably. There may be a mild steel jacket that still attracts a magnet or possibly a steel core in the centre that wouldn't necessarily show by crushing a bullet (or do you mean cutting it right through?)