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I would stay away from lead if you are making gun powder. That smoke that surrounds black powder ignition is not good for you. Fine particles of lead suspended in that smoke would be hell on your lungs etc.. i use a tumbler to get crud off of coins taken from the sea. Beach sand won't work well with water to do the job. But the sand at the oceans edge which is coarse makes a great scrubbing agent. Maybe some aquarium gravel would work to reduce some objects in size. Commercial media is often hell to work with.
You might want to try a nylon type of nut with the steel threads. I believe they are called nylock nuts.
I have lashed with inner tubes and they can be quite strong. Usually, you'll need more inner tubes than you would expect to use.
In the US, it is usually called zinc shakes. It may be mild and a few minutes walking it off might do. But it can be quite serious and I am in favor of calling an ambulance just in case. I really doubt that anyone become immune to it. I suspect that most people who weld get smarter and use better ventilation after they get a mild dose of zinc shakes.
Spring benders are used on the outside of small tubing to control the bend. The spring is the bender. For high-quality work, a mixture of water and a bit of dish soap is frozen inside a tube before it is bent. That keeps it round. Sand can be used but must be tamped down in the tube in such a way that it has no place to move about So it is best to seal the ends, then bend, then remove the sand. But the old time way may be the best way. Use caution and fill the tube with molten lead. When it cools make the bend. Then heat the tube to capture the melting lead into a container. Molten lead is best handled by adults who have some experience as you can be seriously injured or killed.