A little more research and I was able to answer my own question. This is from AAC's website.
-The 9" and 12.5" AAC 300BLK Uppers are designed for use with a carbine receiver extension, standard GI Buffer Spring, and an H2 Buffer.
SGT U.S. Army Special Operations Command, Ft. Bragg, NC (2002-2006)
Operation Iraqi Freedom Veteran (2004)
"Democracy is two wolves and one sheep voting on what to have for dinner. Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin.