There's two problems with brass/bronze that makes a steel liner mandatory for safety.
First, brass and bronze have very low tensile strength and poor resistance to shock loads. Brass and bronze have a tendency to SHATTER under shock loads - good for a grenade but poor for a gun!
Second, in building a cannon, you want materials of KNOWN properties so you can depend on its strength and mechanical characteristics. Brass and bronze are not manufactured to provide consistent strength, impact resistance, etc. - they are manufactured to control other properties that are more desirable in thier normal application.
Although you can pick up a piece of junkyard steel for a liner, I would STRONGLY recommend that you pay a little more for a piece with KNOWN properties.
I used 1061 structural steel (seamless) tube for my liner. Knowing the liner was strong enough to handle the loads, I used "bearing bronze" for the barrel because of its machinability and beautiful mirror finish.