That's half of the M16 magazine problem. The worst flaw is that the *curved* 30-round magazine has to enter a straight-sided magazine well, so has to include a straight-line upper portion combined with a curved lower, a result of Gene Stoner's original design meeting the requirement of having a 20-round capacity. Why? because the M14 and BAR that preceeded the AR-15 designs had 20-round mags, so that was what Army Ordnance specified.
When American soldiers armed with testbed AR15s and early production M16 rifles that would empty a full 20-round magazine in a single burst in less than a second began to encounter NVA and Viet Cong troops with AKs with 30-round magazines, the difference mecame a mattter of survival. The most immediate answer on the American side was to tape two magazines together, one up and one down. With the early light aluminum *waffle* magazines for the M16, this had two additional results, one being that the additional weight bent away the magazine's metal around the mag catch, resulting in misfeeds from a ruined magazine; the waffle mags were subsequently removed from service. The other problem was that the magazine catch of the rifle could be bent enough to prevent reliable feeding even with a single new magazine. One other *field fix* sometimes tried was to modify the magazine of an AK47 to fit an M16, though the magazine catch limitation meant that was just a stopgap.
When Stoner developed his M62 machinegun, which could be fired in either magazine-fed or beltfed configurations, he designed a new magazine for it, omitting the straightside magazine well of the AR15/M16/M16A1/A2/M4 and allowing a magazine that was of curved form for its entire length. When the Israelis combined bits of the FN-FAL paratroop rifle, the AK and the Finnish Valmet m/62 into the 5.56 Galil, design, the first test versions used Stoner 62 magazines, which will fit and feed in the Galil's produced today, except for the ones that have an accessory *adapter well* to let them use dirt-common M16 magazines.
Likewise, when H&K designed their HK33 series 5.56mm rifles based on their roller-locked 7.62mm G3, they used a new magazine with an all-curve profile. As did the Steyr AUG, the followon H&K G36, The first versions of Britain's L64/SA80 bullpup, and most every other 5.56 rifle I can think of that doesn't use the M16 magazine. The one exception that comes to mind was the first versions of the French FAMAS bullpup, which had a rectangular profile 25 round magazine; the current production FAMAS G2 uses the M16 magazine.
The two on the right are the 20 and 30-round magazines for the Ar15/M16/M4:
