If it is below the bore so the bullet does not strike the attachment, it will fire without significant interference, however, the muzzle flash will spread fouling all over it.
The closer it is to the flight of the bullet the more likeli it is that the attachment could cause the bullet to fly erratically. We are talking less than a 1/4 inch below the flight of the bullet. Obviously a big 12 ga jag sticking up into the bore line is dangerous. But when it is just below, the attachment can cause the gases escaping from the muuzzle to spread at less than full 360 degree radius. that open spot in the 360 degree radius can pull the flight of the bullet slightly off. Think of it as being similar to having an off center crown on the muzzle. It won't necessarily cause loss of accuracy, but can if close enough under the bore. (And when there is a deer, you missed, give you the technical excuse you need for your buddies)