An upper parts kit contains the small components needed to complete a stripped upper receiver: the forward assist assembly, ejection port cover (dust cover), and associated springs and pins. Some kits also include the barrel nut and other installation hardware.
Kit Components
Standard Upper Parts Kit typically includes:
- Ejection port cover and spring
- Ejection port cover C-clip
- Forward assist assembly
- Forward assist spring
- Forward assist roll pin
Enhanced kits may also include:
- Barrel nut (specific to handguard system)
- Crush washer or shims
- Gas tube roll pin
- Handguard mounting hardware
Component Considerations
Forward Assist: The plunger assembly that allows manually pushing the BCG into battery. Quality forward assists have properly serrated faces and strong springs. Some builders omit this entirely (slick-side uppers) as it can force malfunctions worse if used incorrectly.
Ejection Port Cover: Keeps dust and debris out of the action. The spring should snap the cover open smartly when the BCG cycles. Quality covers have good finish and proper fit without being too tight.
Quality Indicators
Even these small parts benefit from quality manufacturing:
- Proper heat treatment on springs for longevity
- Good surface finish on the forward assist for smooth function
- Correct dimensions on the C-clip and pins
- Durable finish on the ejection port cover
When to Buy
If building from a stripped upper, you'll need these parts unless going slick-side. Many builders purchase complete upper parts kits from the same manufacturer as their upper receiver for consistent finish. If you're buying a complete upper or assembled stripped upper, these parts are typically already installed.
Slick-Side Alternative
If omitting the forward assist and ejection port cover for weight savings or aesthetics, ensure your stripped upper is designed as a slick-side (no forward assist bore or pivot holes). Converting a standard upper to slick-side requires plugging holes.