most acids don't really dissolve flesh very efficiently unless it's piranhas solution which is near pure sufuric acid mixed with 30% solution of hydrogen peroxide but that is highly exothermic and the resulting gas build up would likely cause the drum to explode. A caustic base like sodium or potassium hydroxide would work faster but you'd still have to deal with bones. Now the infamous hydrofluric acid will eventually dissolve bones but it takes a good while so hopefully your barrel is out of sight for a long time and I wouldn't want to fuck with HF unless you have good skin protection as you have a good chance of being poisoned by a small amount of the stuff and dying from a heat attack caused by hypocalcemia.

Fuming nitric acid (66% or higher purity) is good because it attacks human flesh rather vigorously but again it's rather dangerous to screw around with as unless you're wearing butyl rubber it will not only eat right through most gloves but actually set them on fire.

There's also super acids like fluroantimonic acid that are several trillions of magnitudes more acidic than "normal" acids but who wants to fuck around with that kind of shit? The same kind of assholes who think it's fun to play with dioxygen difluoride and sulfur trioxide.