Yes, zinc is ideal because it is fairly easy to work with and is nearly identical in weight to iron. When I was at Camp Grayling in July this year everybody I talked to was using zinc ammo from the big smoothbores & rifled guns right on down to the mortars.
Lead is still viable in certain cases because it is still plentiful, inexpensive, and casts at much lower temps than zinc (your regular lead melting pot will not melt zinc). Lead balls actually work very well in a properly made full size 3" Ordnance Rifle or 2.9" Parrott (smoothbore). The lead ball ends up weighing just under 6 lbs. and with proper windage will develop FAR less pressure than a 10 lb. bolt with sabot. It is very accurate too. In fact, using a custom made round shot mold shooting at 600 yards this year our rifled Parrott had very little advantage over our smoothbore Parrott, both were consistently hitting the 600 yard target. I was actually quite surprised how well the smoothbore did.
However, as already mentioned above by others, I would not recommend using lead over zinc, iron, or steel in a smoothbore piece such as a 3 Pdr. to 6 Pdr. cannon as the weight will exceed the original load. This is not the case in an originally rifled piece and that is the exception. I know the N-SSA people will disagree with me, but then again they do not shoot service loads or long distance so light loads work fine for them at 100 or 200 yards.
The key to casting your own be it lead or zinc, is you must start out with a quality mold. Fishing sinker rounds will not cut it.
As far as recovering ammo, the smoothbores we shoot have a major advantage as they are very easy to find laying near the target area on the backstop. However, my rifled Parrott bolts end up being buried approximately 5 feet deep in the mountainside and for all intent and purpose are a total loss. This is why I am no longer interested in shooting the big rifled guns at anything less than 1,000 yards.
Cheers~ Anthony Variz