The practical rate of fire of an automatic weapon is also affected by the durability of the weapon system's barrel. The largest problem with firing a weapon at the cyclic rate is the likelihood of catastrophic failure of the barrel (i.e. the barrel melts due to the heat generated by the rounds passing through it). Even with regular magazine changes most every automatic weapon will overheat its barrel within minutes if fired continuously at the cyclic rate. For this reason nearly every automatic weapon employed by the U.S. military has interchangeable barrels that can be swapped out within seconds so long as the weapon has fixed headspace and timing.

The U.S. military breaks down the rate of fire for its automatic weapons into three categories: sustained, rapid, and cyclic.

1) The "sustained" rate of fire is the ideal rate for the weapon system. At the sustained rate of fire, with barrel exchanges at the recommended intervals, the weapon should last all day. This is always the lowest rate of fire for any weapon.

2) The "rapid" rate of fire typically applies only to medium to heavy weapon systems. This rate is used when the weapon needs to be fired at or around its limitations due to heavy offensive pressure on the team, squad, platoon, etc. Recommended barrel exchanges become more frequent and chances of the weapon system experiencing stoppages, malfunctions, or even catastrophic failure become greater.

3) The "cyclic" rate of fire should be theoretical only. This is absolutely the maximum rate of fire at which the weapon will ever operate. Determined partly by the design and machinery of the weapon, it is also based on time taken for magazine and barrel exchanges performed by someone who is an expert with the weapon. Firing any weapon at the cyclic rate will all but guarantee catastrophic failure of the barrel within minutes. Catastrophic failure of the barrel is what is commonly known as a "Very Bad Thing" since it means there is a very good chance the weapon will explode in your face. Imagine having your face less than a foot away from a grenade as it detonates. As stated previously, theoretically no weapon should ever be fired cyclic, but that doesn't mean it never happens. When the chances of being overrun start to beat out the chances of melting the barrel, rules and guidelines start to lose their meaning. This is why the term "going cyclic" is synonymous with "FPF" (Final Protective Fire), or "the fit has hit the shan".

Here are some examples of rates of fire for standard U.S. military automatic weapons (listed in "rpm" for "rounds per minute"):

M-16A2 Service Rifle (5.56 MM)

  • Cyclic: 800 rpm
  • Sustained: 85 rpm

M-249 Squad Automatic Weapon (5.56 MM)

  • Cyclic: 725 rpm
  • Sustained: 85 rpm

M-240G (7.62 MM)

  • Cyclic: 650-950 rpm
  • Rapid: 200 rpm
  • Sustained: 100 rpm

MK-19 (40 MM)

  • Cyclic: 325-375 rpm
  • Rapid: 60 rpm
  • Sustained: 40 rpm

M2 (.50 cal)

  • Sustained: < 40 rpm
  • Rapid: > 40 rpm
  • Cyclic: 450-550 rpm