The mags are 40-rounders, one mounted in the weapon, with two more inverted and duct-taped on either side of the first one.
Oooooh, scary!
Home Depot sells packages of velcro tape, about an inch wide and 4" long. You can back-to-back a whold herd of 30 rd Mini-14 mags with that stuff - make em all the same... put a strip of fuzzy on the bottom of the side, then a strip of hook above it, then a strip of fuzzy, then a strip of hook. Do both sides. If they're all the same, you slap them together one up, one down, one up... I carry them in my shooting bag in groups of six together. Split em into twos on the range or woods.
I'm sure something similar would work for the AR... you'd have to make sure to allow enough neck on the mag to allow it to slide into the mag well.