ATF released a 14-point clarification on what makes a 80% lower. In short, no holes or dimples for selector, trigger, or hammer parts, and no fire control cavity means 80% lower
Link: http://www.atf.gov/content/contact-us/pressroom/receiver-blanks-Q%26As
The Firearm Blog has photos from Ammoland and the whole 14 points
Link: http://www.thefirearmblog.com/blog/2014/11/11/atf-releases-faq-80-percent-receivers/
From: The Firearm Blog, ATF
