before the powder charge is placed into the breech. Large-caliber naval guns are traditionally of the "BL" type. Lacking a cartridge case to seal the breech, these guns require a complex and strong breech mechanism to safely seal the breech during firing. "QF" guns were a later development, usually of small to medium caliber, and came about from the need for rapid firing guns to counter torpedo boats and other small, nimble threats. A QF gun fires metallic-cased ammunition, generally with the shell fixed to it, as a single unit, just like a firearm cartridge. This allows a simpler breech mechanism, such as a sliding block breech, which when coupled with the easier ammunition handling and ramming procedures, allows a much higher rate of fire. Larger guns are most often BL guns, because the ammunition is so large and heavy, that even adding a metallic cartridge case would add a great deal of weight and make handling more difficult, even if the shell and propellant were loaded separately. There have been exceptions, however; some German naval guns during WWII used fabric powder bags with a thin brass base section, to provide obturation to the breech without adding too much weight, similar to a modern plastic shotshell with a "low-brass" base for sealing and extraction. See also Driving band Gas check Minié ball Glossary of firearms terminology References Ammunition Firearm terminology Seal rings