Many scuba weights are zinc or some terrible inconsistent alloy made from scrap with a lot of zinc in it. Since the only thing that really matters is the weight, they would make scuba weights from anything that they could melt. If they're really hard, I'd be suspicious of zinc. The way I normally tell is by melting the suspect metal at a low temperature; just high enough for lead. An electric burner is good for this since it doesn't get as hot as quickly as a propane burner. Zinc wont melt but if there is zinc mixed with lead you'll end up with a metallic sludge with a consistency similar to a slushy from a convenience store. There is no simple way to separate the metals at home.
If it is lead, go ahead and cast. If you're casting for most pistol cartridges I wouldn't even bother worrying about pressure since pistol cartridges produce so little relative to rifle cartridges. The charts out there can be helpful sometimes but in my experience there are far too many variables involved in cast bullets for any chart to really work. You need to go from experience and trial and error. Roughness of bore, sharpness of rifling lands, depth of lands, hardness of lead, pressure curve, twist rate, velocity, powder flame temperature, barrel length, lube type, alloy composition, throat dimensions, diameter variations in the length of the barrel, gas check or not, and many other factors all effect the performance of cast bullets. The charts work most of the time, around 60% in my experience, but they are wrong quite a bit because of factors with a specific firearm they can't compensate for.
