Editing Colt AR-15 (section)

===Upper receivers===
The upper receiver incorporates the hand guard, the charging handle, the forward assist, the gas operating system, the barrel, the bolt and bolt carrier assembly. AR-15s employ a modular design. Thus one upper receiver can quickly and easily be substituted for another. Upper [[Receiver (firearms)|receivers]] are available with barrels of different weights, lengths, calibers, and rail systems with various sights and accessories. The standard AR-15 rifle uses a {{convert|20|in|adj=on}} barrel. Although, both shorter {{convert|16|in|adj=on}} carbine barrels and longer {{convert|24|in|adj=on}} target barrels are also available.

Early models had barrels with a 1:12 [[Rifling|rate of twist]] for the original .223 Remington, {{convert|55|gr|adj=on}} bullets. Current models have barrels with a 1:9 or 1:7 twist rate for the [[5.56×45mm NATO]], {{convert|62|gr|adj=on}} bullets.