The idea that launching lead is somehow more dangerous than launching various other ammunition is highly questionable. Almost all "serious" builds nowadays use metal projectiles of some sort or other. Of particular note in this regard are Miniboy by LeMaudit (mostly brass and aluminum) and the tungsten carbide "dart" rounds currently in development by Jackssmirkingrevenge and Petitlu for use in the latter's 40X hybrid (ThorJack). On a more mundane note, most of the single shot or bolt action pneumatics posted on Spudfiles are designed for use with cast lead slugs. There has been a strong trend toward smaller bore launchers in the past few years, which would help explain the current preference against vegetable projectiles, but this is not always the case - about two years ago, Spudfarm, a Norwegian member, was firing two pound lead slugs at around 300m/s from his 45mm hybrid against large granite and concrete slabs, to great effect.
Lead slugs are not popular in the larger bore sizes simply because most of the largebore guns operate on pressure too low to make much use of them. That would appear to also be the case here. The main safety issue with firing heavier projectiles is that of recoil. The recoil force is simply the force applied to the projectile base, and heavier projectiles will tend to maintain higher base pressure further out the barrel, increasing the possibility of buckling. This must be accounted for, and I hope you were testing these remotely before you started hand-holding the launcher - any sort of rupture which occurs near the hand, even if it is caused by recoil-induced buckling and not overpressure, can be very unpleasant.
As to the legal aspect: spudguns are totally illegal almost everywhere. You should definitely NOT ask your local law enforcement about the legality, because you'll never be free from scrutiny again. If you cause no trouble, and no one hears or sees your launchers, there will be no problem (and no bad publicity). If you're using them at public outings or in the company of people who you don't entirely trust, you may be in trouble regardless of what you're using as ammunition. Remember: to the uninformed, a potato gun may as well be a 20mm autocannon. Pick your spectators with that in mind and handle your launchers safely, and there will be no trouble. They cannot prosecute what they are never aware of.