I liked mrm14s comments. Even though we usually teach to have the AR-15 Mags that hold 30 to have 28 rounds and the 20 rounders with 18 if stored for long periods. I must admit I like m14 have had 45 ACP and AR Mags loaded for years and years and never had one fail me when I placed it in the good old 1911 or the AR-15! I think one needs to consider the actual quality of the magazine. And certainly the material they are using primarily thinking of the spring material. With all the cost cutting some companies are doing today some may be using lesser quality parts than maybe those in the past.
I know some of the lesser quality magazines of today if left completely loaded to capacity for a period of time I have even seen the welds give way on the magazine box. Cheaper plastic composition mags that the lips distort or some warping occurred. By the way MagPul makes some of the best polymer mags with no problems. NHMTG makes some of the best metal mags available which are actually used by several manufacturers who then place their own End Plate with logo on the NHMTG Mags. I too have had mags loaded for the best part of 20+ years that worked just fine!
The Constitution is not an instrument for the Government to restrain the people. It is an instrument for the people to restrain the Government!
Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
-- John F. Kennedy