The primary mechanism for testing metal coins is their electrical characteristics. Run a very small voltage through it, measure the resistance, compare it to tables showing the resistance of known coins, with known metallurgical compositions, sort accordingly. This is how home-sorting machines that hobbyists like the OP’s uncle can buy for ~$500 work. Even with the low end of our numbers, we net about $120 per 5 gallon bucket, so this machine pays for itself pretty quickly. It says it can process 18,000 pennies per hour. A quick search said that about 1/4 pennies in circulation are pre-'82, so figure you get 4,500/hr. It takes about 6 hours to make $120, or about 26 hours to pay for the machine, assuming you can get 26 hours worth of pennies to put into it. Most hobbies never turn a profit, so a hobby that pays for itself in a couple weekends and then begins generating net cash is a pretty good hobby. Questions of logistics and legality aside of course.
Also are the scrap people going to buy your pennies given that its illegal to melt them down. The secret service isn’t going to knock on your door if you melt a few buckets yourself, but a scrap dealer that has a policy of doing this could get in trouble. Again, he’s probably safe, but would it be worth the risk for the few extra bucks a year they would make from maintaining such a policy.
Yes and no. I was posting with the understanding that there will be some future date when you can scrap pennies. And, unfortunately, a lot of scrap dealers will happily accept stole property. There are cities, like Baltimore, where metal theft is common. The park in my neighborhood lost all its trashcans one year. Plaques are pried of statue based, and even maintenace covers are sometimes stolen.
Separating out pennies seems like the easy part of this. I’m going to assume you can sell the copper pennies to someone speculating on the copper value in the future (or who is just illegally melting them down, but you don’t know that).
But where do you get enough pennies to make this work at volume and how do you turn the zinc rejects back into actual money? I have to imagine any bank is going to either start charging you more than you make to handle all that nonsense or close your account.
Maybe if you already have a major cash business that brings in a lot of change, you can run your pennies through before turning them in?