... That the thickness or toughness of the plastic was absolutely not an issue.
The sandwich bag material was thin and easily ruptured. The balls would have shattered even if nothing was in the way- the brisance of virtually any explosive (I'm assuming these have some form of black powder in them) is going to be way too much for a paintball. The force applied is going to be such that it'll squash once side of the ball against the inertia of the other side.
While I plan to try again, with different "cushions" to try and moderate the wave front a bit (I suspect the "hand grenade" will wind up the size of a basketball before it's over) I'm already convinced we'd need something far weaker, on the order of a strong balloon pop, in order to propel a reasonably intact paintball.