AR15 anatomy and component names
Understand the major assemblies and the difference between a component, an assembly, and a complete firearm.
Upper and lower assemblies
The upper receiver houses the bolt carrier group and connects with the barrel assembly, charging handle, and handguard. The lower receiver includes the magazine well and supports the fire-control system, grip, and rear support system. The book's exploded views are useful for learning these names, but a drawing of one configuration cannot establish the dimensions of another.
OPEN FULL SIZE ↗Labeled source illustration of upper and lower receivers, rear support components, and controls.
Source image: Real Avid, Top AR15 Mods, ©2021. PDF page 4.
OPEN FULL SIZE ↗Labeled source illustration of the barrel, handguard, and bolt carrier components.
Source image: Real Avid, Top AR15 Mods, ©2021. PDF page 5.A bolt is part of a larger assembly
The bolt and the bolt carrier group are related terms, not synonyms. Likewise, a barrel, its chamber, and its bore refer to different features. Naming the specific feature helps distinguish a question about documentation from a question about a complete assembly.
Use the exact model name
AR15 describes a broad platform family. The book itself notes that parts vary with configuration. When asking a manufacturer or gunsmith a question, provide the actual manufacturer, model, chamber marking, and known service history instead of relying only on a family name or a reference photograph.
Identify the assembly first, then the individual component. Keep photographs and model documentation together.