This is speculation, but I suppose there might be an issue with the front of the unburned powder column slamming into the stationary bullet. There was a thread on the autoloaders section a while back asking what would happen if a standard 1911 pistol were fired when there was an obstruction holding the bullet in place. What happened (much to my surprise) was "not much"; however, the person performing this experiment was careful to explain that if the bullet were allowed to move, even a little, then the effects were likely to be much more dramatic. Half a powder column would not weigh much, but it could be moving pretty fast when it hits the bullet. I wouldn't discount the possibility that it could cause some damage.