As of v. 2.0.46, the vanilla recipe for Piercing rounds magazines changed. This makes the older casting recipe out of step with it.
With liquid casting costs:
Old vanilla: basic magazine [40 iron] + 1 steel [30 iron] + 5 copper [50 copper] => 1x piercing magazine
Casting: 70 iron + 50 copper => piercing magazine (i.e. same cost)
New vanilla: 2x basic magazine [80 iron] + 1x steel [30 iron] + 2x copper [20 copper] => 2x piercing magazine
This would make the new equivalent cost be 55 iron + 10 copper => 1x piercing magazine.