Ok the amount of time you spend making the steel in a 45 gallon drum usefull makes it worthwhile to just buy new steel. I get my steel from an HVAC contractor because they charge me what they pay for it. But if you go to places like that they usually have useable scraps which are good as new steel just smaller pieces. They'll let you have them cheap possibly free.
Drums are made inexpensively, so they are of mild steel. At a guess, hotrolled.
If you get the drum for free, or for an old song, and purpose to make curved pieces of it -- legs and body mostly, I daresay -- it might be about worth it. What everybody else says up above is good advice, though. Avoid the strengthening flutes; cut around those and discard them unless you have a forge and want to practice heating and beating light stock for some reason. Trying to flatten out an entire drum fluting by cold work is a lot of effort for minimal reward -- a reward in ugly, bashed, Mad Max metal. Expect to generate a lot of scrap from this source, and to be removing paint or inner coatings too.
There isn't a lot you can do with the top of a drum either. I made a roundshield of one once. It looked exactly like the top of an oil drum, even with some dishing.
If you do end up getting it, you may want to find out what was in it so you can take any safety precautions needed. Especially if you are going to heat the metal.
I don't know what gauge it is, but it may be good for making spaulders. Just a thought.
Cat
Catherine's Quest is no longer in business. I may open back up at some point in the future. Thank you all for all of your support over these last few years. It has meant the world to me.