Courtesy of AR15.com this diagram helps give you a good idea of the basic components to your AR-15 lower parts kit. While all lower parts kits may not be exactly the same, they will typically have a close version of the image below. You'll find some kits may not include the fire control group, which consists of the trigger and hammer, because it is a cost effective way to purchase a lower parts kit while installing a different fire control group.
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