You are the one that doesn't understand the technology of "Metal Storm". It uses multiple barrels that have multiple charges and projectiles in each barrel. There will definitely be muzzle blast. Lots of muzzle blast.
Or backblast, if it's arranged as a recoilless weapon, venting its exhaust gasses rearward through a central vent past doughnut-shaped projectiles and prop charges. A few years back the Navy did considerable experimentation on *Ring airfoils* and Metalstorm might be developed as a launcher for such projectiles.
But essentially, it's an electrically-fired Roman candle, and that idea has been used before. Interestingly, the Germans had a followon design to their WWII Mk108 aircraft cannon that was to have used a liquid *fuel* propellant, and which would have had multiple projectiles in the barrel simultaneously. The rate of fire could have been well in excess of 20,000 rounds per minute, as designed, but never advancing to the prototype stage, so far as I know.
-archy-/-