Clamping two magazines together gives you an additional 30 rounds right on the gun. The disadvantage is that the top rounds are exposed and can move under recoil. The solution to that is to roll the magazine into the well rather than pushing it vertically upward.
The Lancer Systems Magazine Cinch is designed to work with L5 30 round M16 magazines but works equally well with USGI M16 magazines. For the L5 magazines it has a special aluminum spacer that stops overtightening of the coupling screws and its clamps fit the ridges on the magazine. The Magazine Cinch is polymer and steel construction and is immensely rugged. It clamps the right hand magazine lower so as not to obstruct ejection when magazines are swapped over.
Check the price of the Lancer Systems Magazine Cinch at Brownells, a name you can trust.
You may find the following video on the Brownells Magna-tip story of interest.
Full disclosure: If you purchase from Brownells I will receive a small commission. This will help keep this site online and will not affect the price you pay. Thank you!