How to Build an AR-15 Upper Receiver
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Building an AR-15 upper receiver involves assembling the barrel, gas system, bolt carrier group, charging handle, and handguard into the upper receiver. The process requires proper headspacing, torque specifications, and timing of the gas system components.
How This Affects Your Build
Upper receiver assembly is where most AR-15 customization happens and directly affects accuracy, reliability, and handling characteristics. Unlike the lower receiver which is heavily regulated, upper receivers can be built and modified with fewer restrictions. The upper assembly contains all the pressure-bearing components and gas system timing that determine how your rifle functions.
The Detail
Start with a stripped upper receiver and install the forward assist and ejection port cover if your receiver doesn't come with them pre-installed. The forward assist spring and plunger go in first, followed by the forward assist itself which typically requires a roll pin. The ejection port cover uses a small spring and C-clip that can be tricky to install without the right technique.
Barrel installation requires an upper receiver vise block and barrel nut wrench. Thread the barrel extension into the upper receiver hand tight, then use the vise block to secure the upper while torquing the barrel nut. Most barrel nuts require 30-80 foot pounds of torque depending on the design, and the gas tube hole in the barrel extension must align properly with the upper receiver's gas tube channel.
The gas system components install next, starting with the gas block which slides over the barrel and aligns with the gas port drilled in the barrel. The gas tube connects the gas block to the bolt carrier group and must be the correct length for your barrel and handguard combination. Carbine, mid-length, and rifle-length gas systems each use different tube lengths and affect the rifle's timing and recoil characteristics.
Finish the assembly by installing the bolt carrier group and charging handle, then mount your chosen handguard system. Free-floating handguards attach directly to the barrel nut and don't contact the barrel, while drop-in handguards use the standard delta ring assembly. Test the action by cycling the charging handle and ensuring the bolt locks back when you engage the bolt catch.
Build Impact
Your barrel length and gas system choice will determine which handguard and gas tube you need, so plan these components together rather than separately. Getting the gas system timing right is crucial for reliable cycling, and this is where most first-time builders run into function issues if components aren't properly aligned.