Are you just trying to get the bungs out or take the whole top off? I routinely turn barrels into burning barrels by sawing off the tops. A reciprocating saw and a fresh fine tooth blade, plus an assistant to keep the barrel from jumping makes short work of the top. Blade follows the outer edge quite nicely. Plasma cutter tips are to expensive and if there's anything flammable in the barrel they are not a good idea. You can also get a barrel deheader, but they cost close to $200. Just a giant canopener.
If you just need the bungs out disregard my post, if you can't figure that part out your project is probably over your head anyway.