I have built several 80% ARs- I do not worry about this issue but it may be a problem, under certain circumstances. I could just see some over zealous officer snooping around the one public range I quit going to....
OTOH, for some reason I just don't give a $h!t.... they ARE legal, and ,well, LE needs to keep up on the laws. I guess.
Really, if you are NOT breaking some laws by using it, well, what business is it of theirs to go snooping.
I stand my ground.