I sent the following email to Bushmaster a week ago and they have not responded. If you have an ACR I would not shoot it, and if you do, be aware that it may fire any time the bolt is closed with live ammo. If you Google the Bushmaster ACR and slam fire, you can read about a guy who shot his big toe off with a 15 round burst, and the safety was on! Email: I was very very happy to receive my new Bushmaster ACR Enhanced this week, I waited a year to get it. Today I took it to the range to sight in the weapon. I loaded 1 round of Parvizan 5.56 and shot it, no problem. A precaution I take with all new firearms. I then loaded a full clip of the PRVI Parvizan 5.56 BT ammo and shot the full clip without incident. I changed stations from the 50 to 100yd benches. It setup and fired 4 more boxes of PRVI 5.56 ammo without incident. The problem started when I loaded Hornaday .223 ammo. I released the bolt from the open position using the release button. The rifle fired from an open bolt. After releasing the bolt with the button, the bolt chambered a round and fired it in a single action. I was very surprised by this and immediately unloaded the rifle. I then removed the bolt and examined it to see if there was a stuck firing pin, or some other malfunction. Everything seemed to operate freely and properly. There was almost no gunpowder residue on the bolt, and I examined the locking rings in the barrel and they were clean and clear. I thought that this may have been a single unusual event. I loaded another single round and tried it, and it loaded and fired as expected. I loaded up 15 rounds of Hornaday .223 ammo and l loaded a round into the chamber using the bolt release button. No problem. I fired 6 rounds and no problem was detected. On the 7th round, I pulled the trigger and 3 rounds fired in quick succession. I was very surprised to fire 3 rounds with 1 trigger pull. I immediately unloaded the weapon and again disassembled the bolt assembly and did not find anything unusual. I was curious about the ammo being a problem so I loaded first a single round, then 3 clips of 10 rounds of the PVRI 5.56 ammo. There was not any malfunction or misfire through the 31 rounds. I have the 3 spent cases from slam fire, the original ammo boxes for the Hornaday, and some examples of PRVI cases fired. Upon examining the slam fired cases, there is an indention from the firing pin, but it is not crisp and deep like the 5.56 cases, or the .223. In addition I have 3 unused .223 Hornaday rounds from the box I was shooting from. I will wait for your reply before touching this firearm again. Please let me know what action you would like to undertake? I am very disappointed that I received a firearm with an obvious defect, after all of the fine articles I have read about the ACR. I actively blog on a number of online gun forums. I will give Bushmaster a chance to respond, before I start posting this information online. I am a fair individual, and it is possible that something I did caused the slam fire, since it is a brand new weapon to me, and the first time that I shot it. I have used both types of ammo in my Bushmaster Varminter for the 2 years that I have owned it without a single problem.