Complex sentence: An additional reason for use only in a manually-operated weapon is the fact that the round shoots flame for 3-5 seconds: this would cause a hazard if a shell still emitting flame were to be ejected from an automatic shotgun.
Split into simpler sentences: An additional reason for use only in a manually-operated weapon is the fact that the round shoots at least a little flame for 3-5 seconds. This would cause a hazard if a shell still emitting flame were to be ejected from an automatic shotgun.