You only get these 'weights' in reaaaally cheap plastic springers, I had some in my first ever spring gun back in the day and found some in the stock of a clone L96. It's essentially manufacturing slag I'd imagine, a mixture of many metals, unwanted impurities and other junk that's been cast in to lumps to give weight to the really feather light sub-£50 springers.
Most 'decent' metal airsoft guns are a very cheap aluminium/zinc alloy as mentioned; some use proper aluminium and steel for the receivers and barrels etc but that's a minority rather than the norm sadly. Not like it's even hard to make a gun with steel, JG and CYMA have done loads of them.