I saw some of them around, but did not use one when I was in Viet Nam. A lot of trial and error file work to get them to fit and and the results depended on when the guy cobbling one together was satisfied with how well it worked in his rifle. I did see one that was held in entirely by friction, no attempt was made to cut a notch for the magazine catch. I can't remember who had it, but I do seem to think it was some larger base camp, like Bien Hoa or Long Binh. I also have a low opinion of "jungle clips," just too easy to fill the weapon full of mud and crap. Admittedly, on the tank, hauling lots of extra small arms ammo was easy, but the risk of long and short round related stoppages made the infantry's habit of wearing machine gun belts like a Mexican bandit look dicey to me.