What happens when 30 round magazines are not enough - you design 40, 50, 60, 100 round beasts. Magpul took the route of simply extending the PMAG design to hold an additional 10 rounds, nothing revolutionary.
I decided to see how the 40 round PMAG performed at the range, so I added it to my SIG556 test sampling. The PMAGs fit the SIG556 very well, I've never had any issues with fit, locking, feeding or any failures of any kind - the 40 round PMAG continued this trend. The magazine, while a poly composite material does get heavy when fully loaded and paired with the already heavier SIG556 (than an AR or M4). My only concern is the wobble of the 40 round PMAG when locked into the lower, it doesn't cause any issues, but my fear is that any "play" with regards to firearms will be an issue at some point, especially the Swiss designed SIG556, a platform that takes full advantage of tollerances.
I can't say I'd outright field the 40 round PMAG with both of my SIG556 platforms over the 30 round options from Magpul, HK, or Troy. My question is more along the lines of how well does the new Magpul drum magazine, of the X Products drum fit, and does it become a better option than the extended profile you gain with the 40 round PMAG? We'll see, but for now I'll keep my 30 round magazines and keep the 40 round PMAG for an eventual AR platform test.