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Types of AR-15 Upper Receivers Compared

With so many pre-built uppers out there (and the obvious demand for all kinds of lower receivers) many AR-15 owners and builders don’t give much thought to the type of stripped upper receiver they want for their rifle or pistol. After all, its only purpose is to mount the barrel and house the bolt carrier group, right? Except the AR-15’s stripped upper receiver is more important than that, and there quite a few variants with different features out there. Today, we’re going to compare them all.

The AR-15’s 5.56/.223 uppers are just a few of the many variants out there. We’ll even cover the AR-10 and LR-308, as well as what upper receivers are shared among different calibers, like 9mm and .308 Winchester.

Types of Stripped AR-15 Upper Receivers

A1 Upper Receiver

Features

  • Integrated carry handle
  • Tear drop forward assist
  • A1 rear sight with windage adjustments

The A1 receiver is the oldest type of stripped upper available today. It was designed in the 1960s and introduced with the first run of ArmaLite-15 rifles, later adopted by the U.S. Military as the M16A1 for use in Vietnam. The original rear sight features an aperture with small and large openings, and windage adjustments with a rotating drum:

The rear sight and carry handle are integrated into the upper receiver. The typical A1 rifle featured a light-profile 20″ barrel with a 0.625″ diameter, 1:12 twist rate, and standard rifle barrel extension:

A2 (and A3) Upper Receiver

Features

  • Integrated carry handle
  • Rounded forward assist
  • New shell casing deflector behind dust cover
  • A2 rear sight with windage & elevation adjustments

The M16A1 wasn’t without its drawbacks. Soldiers and Marines complained about many of the rifle’s features until military leaders ordered a re-design in the 1980s. The upgrade M16A2 stripped upper receiver was born and it baked in plenty of new enhancements that made the rifle more accurate and favorable to shoot. The A1 receiver allowed ejected shell casings to fly near the face of left-handed shooters, risking injury from hot brass. So, the A2 upper included a new shell casing deflector just behind the dust cover. The rear sight was upgraded to afford elevation adjustments (up to 900 meters). Its windage drum was replaced with a more reliable positive-click dial, too:

The carry handle was still left integrated into the receiver itself, but the tear-dropped forward assist was rounded off. The typical A2 rifle sported a 20″ barrel with the same 1:12 twist rate. Field reports stated the A1’s thinner barrel lost accuracy when it got hot, and it struggled with the heavy 40mm grenade launcher often secured to the barrel and handguard for Infantrymen. The barrel diameter was beefed up to 0.750″ toward the muzzle, and left thinner toward the receiver to save weight:

(There is no “A3” upper)

Although the M16A3 is a very real weapon, it does not have its own, unique version of the AR-15 upper receiver. The A3 is simply a fully automatic version of the M16A2 (which is three-round-burst only). The A3 uses the same upper receiver with a fixed carry handle and integrated rear sight aperture.

A4 Upper Receiver

Features

  • New flattop with Picatinny/Weaver rail
  • Low- and high-rise versions available

As optics and technology advanced, the A2/A3 upper receiver’s aging carry handle and rear sight apertures limited the AR-15’s ability to shoot at greater distances with magnification. The entire receiver was revamped and made into a “flattop” configuration and dubbed the A4. It features a Picatinny or Weaver rail that allows for any optic (like the Trijicon ACOG or Aimpoint CompM2 and CompM4) to be mounted. The U.S. Military still needed good ole’ iron sights, so the rear sight was also upgraded yet again, this time made detachable:

The A4 upper receiver, when used with the M16A4 in service today, sports the same old 20″ barrel. The A4 upper receiver is also used for all current-issue M4s, which sport a 14.5″ barrel with a government profile and 1:7 twist rate. The M4 can more accurately shoot heavier rounds (up to 80 grains). The slower 1:12 twist rate found in the M16 barrels favors lighter (55-grain) bullets:

.308 AR Stripped upper Receivers

AR-10 & SR-25 Upper Receiver

Features

  • Extended ejection port for longer cartridge
  • Flattop configuration for long-range optics
  • Re-shaped receiver is incompatible with commercial .308 lowers

The big brother to the M16, the AR-10 was developed by ArmaLite as a battle rifle in 1956. It, and its modern cousin, the SR-25 made by Knight’s Armament Company, are still in use today as Designated Marksman Rifles (DMRs) and they chamber the 7.62x51mm NATO cartridge, equivalent to the commercial .308 Winchester. The AR-10 and SR-25 use a proprietary stripped upper receiver that looks like the AR-15 upper, but it’s longer, heavier, and re-shaped. The ArmaLite AR-10 and SR-25 are two of the only current-production military rifles that can be legally purchased by a civilian on the commercial market without special paperwork:

IMPORTANT:

The AR-10 and SR-25 are not the same as any other AR rifle chambered in 7.62x51mm NATO or .308 Winchester. The official AR-10 is a licensed weapon solely owned and manufactured by ArmaLite, and the SR-25 is exclusive to KAC. While they operate like any other commercially available rifle chambered in .308 (and they use nearly all the same parts), important differences in the shape of the stripped upper receiver prohibit either receiver from being used in other .308 rifles. The back of the upper receiver is angled, while commercial .308 uppers are rounded. Compare the LR-308 receiver’s shape below (circled in red) to the AR-10 receiver above.

LR-308 (DPMS/Freedom Group) Upper Receiver

Features

  • Extended ejection port for longer cartridge
  • Flattop configuration for long-range optics
  • Rounded receiver incompatible with AR-10/SR-25 uppers

The LR-308 upper receiver is what most builders buy when they say they’re building an “AR-10” or AR chambered in .308. This commercial receiver was developed and brought to the market by DPMS, a firearm retailer founded in 1985 that originally machined M203, M4, and M16 parts for the U.S government. DPMS saw opportunity to expand the .308 line of black rifles for the consumer, so they came up with a new receiver design that was compatible with AR-10, SR-25, and .308 parts. But the design wasn’t owned or restricted by either manufacturer.

Today, these commercial .308 uppers are called “DPMS 308” uppers or “Freedom Group” uppers. Freedom Group is the parent company of DPMS, Remington, and Bushmaster, all of which produce .308 ARs and parts.

Dedicated Pistol & Custom Upper Receivers

(You thought there were just one or two receivers, didn’t ya?) The list continues. The AR market is rapidly expanding every year, and builders and companies are shoving new calibers into the black rifle platform. The pistol-caliber AR-15 carbine, often chambered in 9mm Parabellum, has become quite popular. While conversion kits exist for those who want to transform a regular 5.56/.223 gun, the market now offers dedicated and custom upper receivers:

Like this dedicated 9mm upper. It ditches the AR-15’s design entirely, removing the dust cover, shell casing deflector, and re-sizing the ejection port to accommodate the smaller cartridges. Other dedicated uppers exist for larger calibers like .458 SOCOM, .450 Bushmaster, even .50 BMG.

Summary

There a ton of AR-15 upper receivers to pick from. The classic A1 receiver follows the AR-15’s original design, while the latest M16A4/M4 receiver has largely taken over the commercial market as the receiver of choice. There are also new .308 upper receivers – commercial options from DPMS – and original variants used by the military, like the ArmaLite AR-10 and Knights Armament SR-25. If you’re building a dedicated pistol-caliber carbine, you can find a dedicated receiver to finish your build, too.

 

Summary
Types of AR-15 Upper Receivers Compared
Article Name
Types of AR-15 Upper Receivers Compared
Description
This full guide compares all the AR-15 upper receivers: A1, A2/A3, A4/M4, AR-10, SR-25, LR-308, and others. We break down the important differences between each receiver, too.
Publisher Name
https://www.80lowerjig.com/
Publisher Logo

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80Lower Jigs, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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