Long story short have sons ACR which has issues with magazines not feeding jamming. So bought the drum magazine tighter fit an has fixed issues had. Than due to a fuck up with my M249 SAW not being v3 ended up buying an gen 9 m4. Was out back with my boys ACR chewed some gears an pinion head. So we tested to see if mags would swap out. An discovered the mag from the m4 an spare I bought fit ACR PERFECTLY an tested once got ACR fixed an ran it with 11.1v an no feeding issues an a better fit than the original ACR mag. Than threw ACR mag into the m4 an again perfect fit plus larger capacity than the m4 mag.
I have found that also with the Gen9’s. The Gen9 expensive mags don’t fit as good as those generic Pmag’s I think they are called. The ones with the ribs like you have which fit gen 8/9/10 are better. Good find mate.
Yeah I was happy when I realised. As my sons ACR already has the drum mag for it. An than now with the ACR mag fitting an being larger capacity than the normal m4 mags I’m gonna switch to them for my m4. Plus the ACR mag looks mean on the m4. An vis versa with the m4 mags on the ACR.