If the drum contained lube-oil, fuels, or other potentially hazardous liquid substances, then it should have a closed-top with bung openings (not many, if any, are shipped with removable covers/lids). Lube oil is combustible, and will ignite at much higher temps than fuel oils, and especially flammables such as gasoline. We used to cut tops off of old lube-oil drums with a high-carbon steel blade (actually, an old car leaf-spring with a cutting edge ground on one end), by punching a slotted hole and then drive the blade with a hammer along the desired line of cut. It's difficult at best to make cuts with any degree of accuracy, but it works fine just to remove the top. A good option to the hammer-driven home-made blade is an air-hammer with a sheet metal cutting tool, if you have this equipment...noisy, but easy, and straight cuts aren't too difficult to achieve. These are a low-temp cutting methods, but can create sparks, so is not advisable with flammables such as gasoline, and can actually be a bit risky with fuel oils (such as #6) on a hot sunny day, as they have a much lower flash-point temp than lube oils and can begin to flash-off vapors if ambient temps are warm and the drum is in direct sunlight (even more so with dark colored paint).
Keep in mind that steel is porous on a microscopic level, and can absorb liquids (especially lighter, thinner, low-viscosity fluids) into it's pores. When heat is applied, these liquids can vaporize and release from the metal, and if the absorbed material is flammable/combustible and air/oxygen is present, could ignite, even in the absences of spark of flame. An oxy/acetylene or oxy/propane torch can easily provide the oxygen source for this to happen. The use of a cutting disc/wheel can also introduce air into the space, as well as generate enough heat and sparks to provide an ignition source.
For flammables (such as gasoline), I would advise using high-temp internal steaming with the drum placed on it's side and both bung covers removed (bungs vertical in orientation to each other) to allow for venting of steam at the highest and lowest points as well as liquid drainage would be the safest method for liquid/vapor residue removal prior to cutting into the drum. The heat from steaming will remove residue from the pores of the metal during this process as well. Any free liquid residue should be drained into a receptacle prior to steaming. If this is not an option for you, you could fill the drum with water and cut it open while with the water is still running to keep the space filled, as this will displace all vapors and possible explosive vapor/air mixtures out of the space. Don't wet-cut with electric-powered tools...ZAP!!!
If the container is well ventilated with positive-pressure ventilation (ducted blower), or natural ventilation through large openings, the risk of explosion/fire is greatly reduced, although the only effective means to determine that the atmosphere in the space is safe for hot-work is to monitor it with a combustible/flammable gas detector (expensive, and not intended for recreational/occasional use). These are intended for professional use, and require training for the user as well as maintenance, calibration and testing for the instrument. My advice would be to steer clear of any drums which may have contained toxic or flammables (gasoline, nitro-methane, etc) altogether.
Toxic substances pose another threat, being they could react with heat or materials used for cutting/welding to form extremely hazardous mixtures, and without the original drum content shipper or manufacturer MSDS (material safety data sheet) for the residue, you can't determine what these hazards may be. This is a huge risk. If you can't make a positive identification of what the "empty" drum contained, you should not take it from anyone, as it could turn into a major liability. Just saying, US federal hazardous materials regulations (and some states as well) are getting pretty dicey nowadays, and if something goes wrong, the owner of the source is held responsible. Even if you don't own it, but it's located on your property, you're ultimately responsible for any damages resulting from an incident. I'm not trying to scare you here...just want you to know, so you can make informed decisions and not jump into something blindly.
What ever you choose to do, be sure to heat the candidate drum with an internal fire after liquid/vapor residue removal with a wood fire (or otherwise) to completely heat the container (hot enough to burn off exterior paint) to release any remaining residue from the metal's pores, either prior to working it into a smoker/cooker, or prior to final prep (applying exterior paint, etc) before seasoning it for use as a cooker.
Come on back if you have more questions or concerns...always happy to help where I can.
Eric
