Upper Parts
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Upper parts finish an AR-15 upper receiver: the forward assist, the ejection port (dust) cover, the barrel nut, the gas tube roll pin, and the springs and pins that hold them. Builders buy them to complete a stripped upper, to build a lighter slick-side setup, or to replace a worn part.
Most are mil-spec and fit any standard AR-15 upper, but barrel nuts usually belong to a specific handguard, so check before you order.
Read our guide: How to Build an AR-15: Parts List, Tools & Steps
Upper Parts: common questions
Do I need a forward assist?
No. It's a backup for pushing the bolt fully closed, and many builders skip it on slick-side uppers to save weight. If your upper has the hole for one, install either the forward assist or a plug.
Which barrel nut do I need?
Most free-float handguards come with their own barrel nut, so use that one. A standard mil-spec barrel nut is only for drop-in handguards with a delta ring.
What does the ejection port cover do?
It keeps dirt out of the action when you're not shooting and pops open on its own when the bolt cycles. A pin and a small spring hold it in place.