The upper receiver houses the barrel, bolt carrier group, and charging handle, and attaches to the lower receiver via the pivot and takedown pins. While not serialized (and thus not regulated as a firearm), the upper receiver's quality directly impacts accuracy and reliability.
Receiver Types
Forged (7075-T6): Industry standard, made by forging aluminum under extreme pressure. Offers excellent strength-to-weight ratio and proven durability. Most mil-spec uppers are forged.
Billet: Machined from solid aluminum blocks, allowing for enhanced features and aesthetics like integrated trigger guards, unique rail profiles, and tighter tolerances. Generally more expensive but can offer improved fit and finish.
Assembled vs. Stripped: Assembled uppers come with the forward assist and ejection port cover installed. Stripped uppers let you choose these components (or omit them for a slick-side setup).
Key Features
M4 Feed Ramps: Extended feed ramps that continue into the barrel extension. Essential for reliable feeding, especially with heavier bullets or non-standard ammunition. Ensure your barrel extension matches (M4 ramps on both or neither).
T-Marks/Picatinny Rail: Flat-top uppers with a Picatinny rail allow mounting of optics and sights. T-marks indicate standardized slot numbering for consistent accessory placement.
Forward Assist: Allows manually pushing the BCG into battery. Traditional feature but increasingly omitted on modern builds as it can force a malfunctioning round into the chamber.
Ejection Port Cover: Dust cover that keeps debris out. Spring-loaded to open automatically when the BCG moves rearward.
Fit and Finish
The upper/lower fit affects both aesthetics and potentially accuracy. Some builders prefer tight fits with no wobble, while others prioritize easy assembly/disassembly. Many manufacturers offer matched upper/lower sets for optimal fit. Look for quality anodizing that's consistent in color and resistant to wear.
Slick Side Uppers
Uppers without forward assists or ejection port covers save weight and offer a cleaner look. Popular for lightweight builds and competition rifles where the features aren't needed. Ensure you're comfortable without these features before choosing this option.