What is a upper receiver
The upper receiver affixes to the lower and holds all of the other parts. You can purchase an upper receiver with all the parts already in place, including the barrel, gas block, charging handle and other components. You can also buy a stripped upper receiver, into which you can install whichever parts you choose. The approximate amount of upper high-end AR-15 that has been approved by ARFCOM is about $1520.
Different Upper Receivers
Do all AR 15 lowers fit all uppers??
Yes any standard lower can be attached to any standard upper. That's the beauty of the AR. In general, all quality forged uppers are essentially the same. The differences come in with things like BCM saying they keep their uppers tighter at the front, with others keeping their tolerances "past tight" so uppers and lowers have to wear in to mate easily, and so on.
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