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.


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

For a long time, everyone and their brother wanted a chrome-lined barrel. They’ll pay extra for it. They’ve heard it’ll last almost forever. They’ve heard it’s the best barrel that ever existed.

And they’re only partly correct.

Chrome lining can protect the barrel from heat and pressure. That’s true.

A good chrome lined barrel can still last for 5,000 rounds before you see any degradation. Maybe more.

However, chrome lining can be uneven in barrels, depending on the manufacturer. The barrels can be SLIGHTLY less accurate than a non-lined barrel (you ARE lining the rifling inside the barrel and therefore there is a slight coating over the lands and grooves, dulling them ever-so-slightly.)

Essentially, chrome-lining is one of those leftover processes that “a guy at the range heard from his buddy at work that his dad always told him…” that chrome-lining was the ONLY kind of barrel you’ll want/need.

This is not to take away from chrome-lined barrels. Like I said, they protect against heat and pressure in an AR. It’s not to say that there are zero benefits to using one.

But there are a LOT of other choices when it comes to barrel treatments. See also Meloniting, Gas Bed Carborization, and Cryogenic Tempering

The bolt holds the round in place ready for you to pull the trigger, which releases the hammer. The hammer strikes the back of the firing pin pushing it through the bolt to strike the primer on the round thereby firing it. The bolt is subjected to the highest amount of stress in your AR.

For that reason, don’t go cheap when choosing your bolt. Rather than go with Carpenter 158 steel for your bolt, FSD recommends 9310 steel. The Carpenter steel is just too brittle. Sure, the 158 is cheaper, but it WILL break – and when it does, your gun is down until you find a new one.