Best AR-15 Upper Receiver Assemblies for the Money  - Pew Pew Tactical
How to Build an AR-15 Upper Receiver: Ultimate Visual Guide - Pew Pew Tactical
Wing Tactical - Spike's Tactical Ejection Port Door, $8.95 (https://www.wingtactical.com/firearm-parts/ar-15/upper-receiver-parts/upper-receiver-parts/spikes-tactical-ejection-port-door/)
Buy the CMMG Upper Receiver Group and more quality Fishing, Hunting and Outdoor gear at Bass Pro Shops.
AR 15 ideas and tips
Wing Tactical - Aero Precision 5.56 Nickel Boron Bolt Carrier Group, $145.95 (https://www.wingtactical.com/firearm-parts/ar-15/upper-receiver-parts/bolts-carriers/aero-precision-5-56-nickel-boron-bolt-carrier-group/)