Asphyxiant gas can smell the odorants, and assigning a different smell to each gas may be impractical. Another difficulty is that most odorants (e.g., the thiols) are chemically reactive. This is not a problem with natural gas intended to be burned as fuel, which is routinely odorized, but a major use of asphyxiants such as nitrogen, helium, argon and krypton is to protect reactive materials from the atmosphere. The dangers of excess concentrations of nontoxic gases has been recognized for centuries within the mining industry. The concept of black damp (or "stythe") reflects an understanding that certain gaseous mixtures could lead to
Gas-operated reloading piston has traveled a short distance. Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports. A gas trap system involves 'trapping' combustion gas as it leaves the muzzle. This gas impinges on a surface that converts the energy to motion that, in turn cycles the action of the firearm. Hiram Maxim patented a muzzle-cup system in 1884 described in though it is unknown if this firearm was ever prototyped. John Browning used gas trapped at the muzzle to operate a 'flapper' in the earliest prototype gas-operated firearm described in . The Danish Bang rifle used