FAQ: AR-15 Upper Receiver Questions Answered

An AR-15 upper receiver (stripped or assembled) measures roughly 7.5–8 inches from the front barrel threading to the rear where it mates with the lower receiver. That measurement refers to the receiver body only—not the barrel, handguard, or other attached components. Most mil-spec receivers follow consistent sizing, but always verify dimensions to ensure a wobble-free fit with your lower.

Start by securing the upper in a padded vise block to protect the receiver. Install the forward assist and ejection port/dust cover with their pins and springs (these are pre-installed on most assembled uppers). Thread the barrel into the receiver, torque the barrel nut to spec, then install the gas block and gas tube, making sure the gas port is aligned. Use proper punches, a torque wrench, and follow the manufacturer’s torque and timing specs. If you encounter resistance or uncertainty, stop and consult a qualified gunsmith.

“Best” depends on your purpose. Key factors are quality construction, mil-spec compliance, and precise machining. Forged receivers in 7075-T6 aluminum offer an excellent balance of strength and weight; billet receivers provide unique aesthetics and sometimes tighter tolerances. A Type III hardcoat anodize improves wear resistance. For retro builds choose a carry-handle upper; for modern optics go flat top. Blemished receivers can save money if the flaws are cosmetic and the structural integrity is intact.

To finish a stripped upper you’ll need:
Barrel
Bolt carrier group (BCG)
Gas tube (pistol, carbine, mid-length, or rifle-length to match your gas system)
Gas block
Barrel nut
Handguard (M-LOK or KeyMod or free-float style)
Charging handle
Forward assist (if not already installed)
Ejection port/dust cover (if not already installed)
Optics or iron sights are optional depending on whether you have a flat top or carry-handle upper. Use mil-spec or confirmed compatible parts to avoid fitment issues.

No. The ATF classifies the lower receiver as the firearm; upper receivers (stripped, assembled, or blemished) are non-regulated components and may be shipped/sold without an FFL. Always follow federal, state, and local laws when purchasing or assembling firearms.

No. Most AR-15 uppers are designed for direct impingement (DI) systems. Gas piston systems require uppers with the correct geometry or clearance for a piston and rod. Some mil-spec receivers can accept aftermarket piston kits, but compatibility varies—confirm specifications before converting.

Indirectly, yes. The upper receiver provides the mounting and alignment platform for the barrel, gas system, and optics. Poorly machined receivers or loose fitment can introduce misalignment or wobble, which harms consistency and accuracy. A well-made flat-top upper offers a stable optic platform; a structurally sound blemished receiver can perform as well as a non-blemished one.

Place the upper in a padded vise block. Align the barrel index pin with the receiver’s barrel extension slot, thread the barrel into the receiver, then torque the barrel nut to the manufacturer’s specification. Install and time the gas block and gas tube so the gas system aligns correctly. Don’t force parts—if things don’t thread or align properly, consult a gunsmith.

Use a padded vise block when working on the upper and store the rifle in a padded case. A hardcoat anodized finish resists minor nicks and wear. Avoid dropping tools or components on the receiver and handle parts carefully during assembly. Regular inspection and gentle cleaning help preserve the finish.

You don’t grease the receiver itself, but a light layer of proper firearm grease or oil on the bolt carrier group contact points inside the receiver reduces friction and wear. Avoid over-greasing, which can trap fouling. The exterior rail or carry handle should be cleaned and lightly oiled to prevent corrosion.

Most are forged from 7075-T6 aluminum for strength-to-weight and mil-spec durability. Billet uppers are machined from solid 7075-T6 and offer unique looks and, sometimes, tighter tolerances. Avoid cast aluminum receivers for duty or precision builds—their strength is inferior to forged or billet.

No. AR-15 and AR-10 platforms use different dimensions, threading, and bolt carrier group sizes. They are not interchangeable and attempting to swap them will produce fitment and safety issues. Always match uppers and lowers to the same platform.

Field strip the rifle and remove the BCG and charging handle. Use solvent and a brush to remove carbon from the BCG channel and gas port area, then wipe the interior and exterior with a solvent-dampened cloth. Dry thoroughly and apply a light coat of oil where recommended to prevent corrosion. Avoid abrasive tools that could alter tolerances or damage the finish.