Nuge, lets say you could take a picture in slow motion with tape or ballon over the barrel it would show the pressure would balloon out the material upon firing the firearm, long before the bullet reached the tape or balloon. Both tape and ballons are elastic and flexible, when you push air into them they move forward...no! Then apply this to a solid substance such as mud or hard snow, they do not have the same characteristics and flexible enough to move rapidly ahead of the projectile. While it may be true some mud or debris loose packed (meaning there is chance for air to escape quickly), should blow out with no effect, it isn' t something you should chance. If by chance the balloon or tape dimpled in would it not be assumed that the air created this dimple in the first place and thus it will not create an obstruction. You see the key is elasticity(sp??) in the materials mentioned, if it moves forward from the rush of air, it will not create built up pressure inside the barrel. The main surface is in fact outside the barrel, not in the barrel like mud, snow or debris.....does this make sense. The air or pressure blows out the tape or ballon, not the bullet. With an obstruction the air or bullet can' t escape normally, causing problems