[rqnvuujp] Pipe bombs

It's kind of surprising pipe bombs work at all, as a bunch of the energy of the explosive is spent turning the pipe into shrapnel.  How does it compare with a cardboard or PVC tube packed with the same amount of explosive surrounded by an outer shell of "pre-minced" shrapnel with equivalent mass as the pipe?  Does the compressive effect of the metal pipe enhance destructiveness or lethality, perhaps through higher shrapnel velocity, despite the energy wasted?

Of course, pipe bombs are more convenient (the pipe provides sturdy packaging) and easier to construct: no need to obtain or create shrapnel.