I had a barrel with this bad of red rust. You need to get boiled linseed oil and mineral spirits (Home Depot or Lowes - I even found it at Walmart). Mix 50/50 and coat barrel. When it is wet, use a deer antler cut at a 45 to mimic a chisel. Scrape the barrel to remove the red rust. Use a cotton rag with fresh mixture to wipe away the residue. You can get antler in a pet store, it's sold as dog treats.
This will darken the metal and help loosen the red rust while you scrape at it with the antler (if you have ivory, use it, but the antler works for me). This mixture will also help keep it from reforming.
Warning, mix very little at a time, and don't discard rag in trash. linseed oil has a tendency to heat up when the rag is balled up, and can ignite. Rare, but possible. I lay the rag out to dry and then toss in the weekly pickup. I pour the excess liquid out in the garden (wife wonders why plants won't grow there)