Dictionary.com defines a “caliber” (as it pertains to ordnance) as, “the diameter of the bore of a gun taken as a unit of measurement.”

In a rifled barrel, the distance is measured between opposing lands and grooves; groove measurements are common in cartridge designations originating in the United States, while land measurements are more common elsewhere. Good performance requires a bullet to closely match the groove diameter of a barrel to ensure a good seal.

Rifle_cartridge_comparison

The beauty of the AR-15 platform is the ability to use many different calibers in the same lower receiver.

Wikipedia shares the following calibers as possible for the AR-15 platform:

Rimfire calibers

  • .17 HMR
  • .17 Winchester Super Magnum
  • .22 Long Rifle

Centerfire calibers inch measurement

  • .17 Remington
  • .204 Ruger
  • .223 Remington – .223 cartridges may function in a 5.56x45mm rifle, however 5.56×45 cartridges will produce excessive pressure in a .223 rifle.
  • .243 Winchester Super Short Magnum
  • .25 Winchester Super Short Magnum
  • .30 Carbine
  • .30 Remington AR
  • 300 AAC Blackout (7.62×35mm)
  • .300 Whisper
  • .375 Reaper
  • .40 S&W
  • .45 ACP
  • .450 Bushmaster
  • .458 SOCOM
  • .50 Beowulf
  • .50 Action Express

Centerfire calibers metric measurement

  • 5.45x39mm
  • 5.7×28 FN
  • 6.5mm Grendel
  • 6.8 mm Remington SPC
  • 7.62×37mm Musang
  • 7.62×39mm
  • 7.62×40mm Wilson Tactical
  • 9mm Parabellum
  • 10mm Auto

Calibers which will not feed but which are used in rifles where the receiver functions only as a trigger group

  • .50 BMG