Out of curiosity, I did a quick look-up on powerheads, also called bang sticks, used by divers against sharks. (My thought being that they are in some ways similar, with a cartridge held at the end of a spear in the case of the powerhead, to what the OP suggested.)
Having used bang sticks more than a few times: they DO have a barrel, albeit an abbreviated one, but the shell is completely enclosed and the force of the firing does propel the projectile at high velocity. It's not an "uncontained" deflagration like an uncontained cartridge.
We did experiment with blanks v/s cartridges, and effectiveness was about the same. What we found was that crimped cartridges were easier to keep waterproof at depth as compared to blanks so that's what we used. 12 guage to start, then .223, and then finally .44 and .38 cartridges were used. All worked about the same. Smaller was easier to use and easier on the diver.