This is a video showing what happens when you load a muzzle-loading rifle with DENSE nitro powder using a BULK powder measure. It also shows the effects of a barrel obstruction. I expect similiar results if you load any dense nitro powder using BP or bulk powder measure regardless of whether the gun has composite or fluid steel barrels.
120gr of HS-6 creates a pipe bomb.
http://www.youtube.com/watch?v=gmsBF6CXs18