You need to purchace the correct PMAGs for your ACR. My legacy window mags would not lock the bolt to the rear on the last round fired. So I called Magpul. They told me that it was a known issue, so they redesigned the PMAG. If you buy dirrectly from Bushmaster, you are sure to get the new ones. However, if you choose to purchace elsewhere you can easily identify the new from the old. The new ones have a forrest green follower, the old ones have a grey follower. For those who don't know, the follower is the ramp that moves up and down in the mag. When I called Magpul they said they haven't updated the window mag yet, so buyer-be-ware. The good news is that the new mags will work in everything that takes an AR mag.