How many calibers?
Searching around, I found some interesting calibers that can be used on an AR lower.
AR-15, without bolt modification
.17 Remington
.20 Tactical
.204 Ruger
.221 Fireball
.222 Remington
.222 Remington Magnum
.223 Remington (5.56x45mm)
.223 Remington Ackley Improved
6x45mm
6mm Whisper
6.5mm Whisper
7mm Whisper
7mm TCU
.300 Whisper (.300/221, .300 Fireball)
.338 Whisper
AR-15, with bolt modification
223 WSSM
5.45x39mm (.21 Genghis)
243 WSSM
6mm PPC
6mm BR Remington
6.5mm PPC
6.5mm Grendel
25 WSSM
6.8x43mm SPC
.30 Herrett Rimless Tactical (6.8x43mm case trimmed to 41mm and necked up to .308; the 6.8mm version of the .300 Whisper)
7.62x39mm
.357 Auto
.458 SOCOM
.50 Action Express
.50 Beowulf
AR-15 using a simple blowback operation
.17 HMR
.22 LR
.22 WMR
9x19mm
40S&W
10mm Auto
45ACP
450 Bushmaster- Didn't say if this caliber required a bolt mod or not.
The forum I plagiarized this from went on to list a whole bunch of others, some one of a kind rifles, including the .50BMG. I am talking about calibers which will fit into the magazine well of a mil spec AR lower.
I have no idea what the bolt modification is, can someone explain?
I understand the blowback system ie. no gas tube, and assume it is built into the upper for the required caliber.