Quoted:
So you are telling me that the Dept. Of Energy pistol upper on ebay last year that sold for $700.00 could have been legally mounted on my lower receiver and made into a pistol?
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The DOE upper could legally be mounted on a registered full auto M16 lower, or on a lower with a registered machinegun conversion device (lightning link or DIAS) installed.
It could also be legally mounted on a pre-ban lower receiver which has been registered as an SBR ($200 tax, law enforcement signoff, pictures and fingerprints, long wait).
It could be modified (removal of the flash suppressor and bayonet lug, if present) to mount legally on a post-ban lower receiver which has been registered as an SBR.
It could be legally mounted on a pre-ban AR-15 pistol receiver without modification.
It could be legally mounted on a post-ban AR-15 pistol, provided that the handguards and any other evil features (flash suppressor, bayonet lug, if present), and provided that the assembled pistol is either under 50oz in weight, or the magazine has been made non-detachable.