The harder a metal is, the more brittle it is, making it more fragile in gun barrel application.
A barrel must be able to "breathe", as it were. If you are ever able to view a time lapse film of a rifle barrel being fired, it is much like a snake swallowing its prey. A bulge travels the length of the barrel, until the projectile is expelled. A brittle metal barrel would fail over an unknown period of time due to repeated stress of repeated firings. Like when a metal strap or wire breaks from repeated bending.
When it fails, there would be no warning, such as cracking or bulging you would find in a barrel manufactured from the proper materials. A brittle barrel would most likely fracture into splinters and fragments and become lethal sharpnel. As has been the case when people have attempted to utilize common water pipe as gun barrels.
Having seen silicon and aluminium bronze breaks, I cannot support the use of either in barrel applications. The fractures appear crystaline in nature. And the crystaline nature of the metals, therefore, make them poor candidates for barrel applications.
Yes, I know I repeated myself, but I am trying to be perfectly clear when I say silcon and aluminium bronze are not a choice for making a gun barrel.
CP