A few things. First, insulated wire is always better. But the thin insulation is the best. So find enameled wire.
Second, the more turns the better result.
Third, you can't just spread wires along the length of your core. The closer together wires are, the stronger field they will make.
Then, you also want part with highest density of wire to be closest to whatever you are trying to magnetically influence. Or in other words, if you pack wires closely together on one end of rebar, that's where the magnetic field is going to be the strongest. Other side of rebar will have considerably weaker magnetic field.
Fourth, while more amps is always better for electromagnet, your wire is way too thick. I'm not sure your battery can handle that much current, and more current always ends in more heat. Thinner wires always work better for electromagnets, but make sure it's not too thin, or it won't be able to conduct enough electricity to magnetize the rebar.
All of that said, don't expect a very powerful reaction, because with single battery and only 50 turns, you will not get significantly strong magnetic field.
Trust me, I speak from experience. I constructed electromagnets with way more turns and stronger power supply, and it could still only barely pick up a paperclip. The most powerful reactions I got using very light coil near a strong magnet.
Proper electromagnets use very thick specially designed core with many turns and usually cooling system in place. So don't worry if your reactions seem underwhelming and if your electromagnet heats up rather quickly.