We were issued nothing but Colt 30 rd magazines the entire time I served as an agent. I participated in countless missions and we qualified every three months. We had to throw away a box of magazines every range detail, due to magazine-related failures. Often, rounds would foul on the follower causing spring coil bind or nose-dive into the front of the mag. All of us ran 28 rounds in our mags so we could press the cartridge stack down and check that all was well. Most of us disassembled, cleaned (mag and ammo), and reloaded our magazines after every range detail.
Twenty rounders were never that particular. The Colt SMG magazines are also spotty performers, often spewing the ammo everywhere without warning. Field use exposes the weaknesses of even the best weapons.