AR-15 ALTERNATIVE CALIBERS
Anything Except 5.56/223
Folks have been looking to get a bigger, more powerful round out of the AR-15 for years without the need to go to it's bigger, older and heavier sibling the Armalite AR-10 or DPMS LR-308.
In recent years the 6.8SPC and 6.8spc II have been emerging as the leaders in this quest. Then in 2010 a new well thought caliber was brought to market. That caliber is the 300AAC Blackout, which by most reports is taking the shooting world by storm. Below will be links to dedicated pages that relate to these alternative calibers as well as whatever viable new ones surface in the future.
I will not be going in to the past for calibers that are fading away.
The 6.8 mm Remington SPC was designed to perform better in short barreled CQB rifles after diminished performance from the 5.56 NATO when the AR15 was changed from the rifle configuration to the current M4 carbine. The 6.8 SPC delivers 44% greater energy than the 5.56 mm NATO (M4 configuration) at 100–300 metres (330–980 ft). The 6.8mm SPC is not the ballistic equal of the 7.62x51mm NATO cartridge; however, it has less recoil, is more controllable in rapid fire, and lighter, allowing operators to carry more ammunition than would otherwise be possible with the larger caliber round. The 6.8 mm generates around 1,759 ft·lbf (2,385 J) of muzzle energy with a 115-grain (7.5 g) bullet. In comparison, the 5.56x45mm round (which the 6.8 is designed to replace) generates around 1,325 ft·lbf (1,796 J) with a 62-grain (4.0 g) bullet, giving the 6.8mm a terminal ballistic advantage over the 5.56mm of 434 ft·lbf (588 J). In recent developments (the period 2004-2008) the performance of the 6.8 SPC has been increased by approximately 200 ft/s (61 m/s) by the work of one ammo manufacturer and a few custom rifle builders using the correct chamber and barrel specifications.
300 AAC Blackout
This is a new project from Wilson Combat. As far as I know they are the only source for hardware at this time.
.45 ACP AR-15