What's the difference between an upper receiver and a complete upper?

An upper receiver is just the machined aluminum housing. A complete upper is that receiver plus the barrel, bolt carrier group, charging handle, handguard, gas system, and muzzle device, everything you need to drop onto a lower and start shooting.

What parts of the upper receiver group wear out?

The bolt (especially the lugs and extractor), gas rings, extractor spring, and gas tube. The receiver itself, handguard, and charging handle rarely wear. Barrel life depends on caliber and round count: expect 5,000 to 10,000 rounds from a chrome-lined 5.56 barrel before accuracy noticeably drops.

Do I need a forward assist?

Almost never in practice. It was added to the M16 for combat scenarios where a slightly out-of-battery round needed to be seated. Civilian shooters can go their entire life without using one. Slick-side uppers (no forward assist) are perfectly functional.

What is the barrel extension?

The barrel extension is the threaded, lugged steel piece screwed onto the chamber end of the barrel. The bolt locks into it, not into the receiver. This is why the AR-15's aluminum receiver can be so light: the actual pressure of firing is contained by the steel barrel extension and bolt lugs.

Which parts of the upper are user-replaceable?

All of them. The upper is designed to be modular, you can swap barrels, gas systems, handguards, BCGs, and charging handles without touching the receiver itself. That's why the upper is where nearly all AR-15 customization happens.

Does the AR-15 upper receiver contain any springs?

Only two: the ejection port cover (dust cover) spring, and the forward assist plunger spring. The main action springs, the buffer spring and hammer springs, all live in the lower receiver.

AR-15 Upper Receiver Anatomy: Every Part Explained

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