These guys are "from the Special Forces (SF) community," and say: "The AR-15 is designed so that it cosmetically looks like the M4A1 carbine assault rifle, but it is impossible to configure the AR-15 to be a fully automatic assault rifle."A Letter From The Special Forces Community Concerning The Second Amendment
Depending upon what the SF guys mean by the word "configure," at least some AR-15's CAN be converted to fully automatic.(Doing the conversion legally requires the same amount of paperwork and costs around the same as legally buying a new fully-automatic weapon.)
It even looks like the military discourages the use of an AR-15 type rifle in the fully automatic mode. Most gun owners say that is a waste of ammunition. And there are mechanical attachments which can quickly pull the trigger in 3-round bursts, which seems to be around optimum for shooting and keeping the rifle on-target.