ACRs are the opposite of an AR-15, the upper is "serialized" and is the FFL part.
However they are supposedly extremely-quick-changing and you just pop off the barrel and bolt carrier group (Which I believe, from seeing the pics/videos/etc. are connected) and put on the new one. In the 5.55 to/from 6.8 conversions you just put one it pull the other out thats it.
The lower is just a polymer part that connects to the main "drive shaft" if you will in the upper.
http://www.thetruthaboutguns.com/201...ushmaster-acr/
That article, if you use the browser search function and search for "lower", mentions the AK lower I was talking about. Incidentially the other hits for "Lower" mention the other Lower i described which will take a regular pistol grip , unlike the standard Bushmaster ACR lower which is a big piece of molded polymer plastic.
if you want to read something informative (I'm assuming you don't know much if anything about the ACR let alone the Magpul Masada), I would check out this on the Magpul Masada, which is where the ACR came from. If Bushmaster just massed produced the Magpul Masada it would probably be the hottest thing on the market 2 years running.
http://www.magpul.com/pdfs/masada_technote.pdf
Scroll down and you'll learn how it works and the AK lower.