I was under the impression that a 50lb bottle weighted 50lb...?
As mentioned, a 50 pound cylinder holds 50 pounds of CO2 gas; the weight of the cylinder is in addition to this 50 pounds.
In the lab, we use 50 pound gas cylinders; they are impossible to lift up by yourself. If you don't have a dolly, then you'll have a very tough time moving them.
Also, forget about moving them if you have a carpeted floor.