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When building an AR what are the different parts and kits that are needed?
Stripped lower + Lower parts kit
Barrel
...?
I would start at Brownells and looking at an exploded view of a Colt or DPMS AR. Those are all the parts you need. You can then start to look up the parts and determine what comes in assemblies, kits, etc. As mac1911 mentions, you can go from buying each part individually and choosing exactly what you want or you can buy assemblies that have things pre-configured. All determined by your time, effort, interest, and of course budget.