I have only had a couple of issues with my ACR. I had two consecutive rounds not go off. Pulled the trigger, click, grabbed the charging handle to cycle the action, pulled the trigger, click, dropped the mag, cycled the action. I inspected the two ejected cartridges and there was no primer mark on them.
I put the rounds back into the mag and tried to fire them again. They both went off fine the second time. The rifle also performed flawlessly for the rest of the day(~150 rounds)
I am not sure why they didn't go off, but there was only about 500 rounds through it so far, so it could be part of a break-in process. The rifle fired flawlessly the day before(~200 rounds).
I love the rifle though, it is definitely one of my favourites. I just have to get the barrel conversion done as it is still classified as Restricted.