80% lower fits and is not restricted....
80% lower does not fit and is required to be registered as a restricted firearm once it does. If your 80% lower fits, it's not "80%".
last time I looked the only areas on a 80% lower is pocket for trigger parts and pins, it does fit the upper and is 80%
.... Some have all the machining done except for the trigger pocket. Look at the most recent Dlask lowers and Polymer80 Lowers, all sold as 80%ers and you can fasten a upper to them...
Here is a picture of a so called 90% lower that has the rear takedown pin completed sans the fire group area cavity. As it is you can couple the upper with this lower and have a mock-up of a complete AR.
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The thinking or selling feature of this model of lower is ensure this area is already machined with the correct tolerances for a proper fit. So the story goes.
Here is a picture of the current 80% Dlask lower. I think it should be obvious that you cannot fasten an upper to it as per what Sqr said.
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Funny cause the 80% lower I just bought from them has it machined out and it will fit a upper, it just needs the trigger pocket milled out (+trigger and selector holes drilled).



























