Since blank cartridges generate very little recoil, far less than that produced by a live round, the recoil operation mechanism is not suitable for use with blanks. BFAs used with recoil operated firearms typically replace the locked breech of a recoil operated firearm with a simple blowback system using a restricted barrel, similar to a gas operated BFA. Short recoil operated pistols, the most common type used for self-defense and by police, are typically converted with a simple barrel replacement; the replacement barrel will lack the locking lugs to lock the slide to the frame, and will be built with an adjustable restrictor to control the chamber pressure. On designs with tilting barrels (again, the majority of modern designs are like this) there may also be a provision for tilting the barrel back, simulating the unlocking of the slide — this is visible in US Patent 5,585,589 listed below. One notable exception to the design of BFAs for recoil operated firearms is machine guns based on short recoil designs, such as the German MG 34 and its descendants. These designs use a muzzle booster to add energy to the recoiling parts, and muzzle brakes for these designs simply replace the muzzle booster with one that provides far more boost with blank cartridges. The M2 heavy machine gun, while it does not use a muzzle booster normally, can use a similar muzzle-booster-derived BFA.