Complex sentence: It appears there is nothing inherently wrong with the concept, other than inadequate initial projectile velocity: a round can actually be stopped if a finger or even piece of cardboard is held directly at the end of the barrel, and will burn its fuel uselessly.
Split into simpler sentences: It appears there is nothing inherently wrong with the concept, other than inadequate initial projectile velocity. A round can actually be stopped if a finger or even piece of cardboard is held directly at the end of the barrel; a projectile stopped this way will burn its fuel uselessly.