Maglink coupler - Magpul Industries MagLink Coupler PMAG 30/40 AR/M4

Cool product. Does its job well. Easy to install and keeps the magazines tight and secure. Great way to have 60 rounds ready to go. NOTE: If you are using an Aimpoint PRO and you stagger them as shown in the instructions with the left mag higher, it will hit the mount knob and the right-side mag will not insert. And if you set the right-side mag higher it will block the ejection port. To avoid this dilemma you can attach them at the same height, or flip one upside down. Both have their drawbacks. Setting them at the same height caused the right-side mag to rub on my ejection port cover when it was open, and flipping one mag upside down makes it a bit harder to switch between them, and if you drop them there's a higher chance they'll fall right onto the feed lips, so you'll need to keep an impact cover on one of them.