PMAGs are heavier than USGI.
A standard, loaded, 30-round USGI magazine is 16.7 oz. 20-rounders are 10.2 oz.
PMAG 30 AR/M4 RevM, 17.9 oz.
PMAG 30 AR/M4 Rev M2 MOE, 17.9 oz.
PMAG 30 AR/M4 Rev M2 MOE (with window), 17.8 oz.
PMAG 20 AR/M4 Rev M3, 12.6 oz.
PMAG 30 AR/M4 Rev M3, 17.8 oz.
PMAG 30 AR/M4 Rev M3 (with window), 17.8 oz.
PMAG 40 AR/M4 Rev M3, 23.3 oz.
PMAG D-60 AR/M4, 44.4 oz. or 46 oz. with M855 according to Magpul's own specs.
PMAG 30 AK MOE, 24.0 oz., this is lighter than a standard steel AK magazine by about 5 oz.
Those ounces add up if you don't watch them.
One reason this might matter is if I give PMAGs the Malf bonus I talked about earlier. It changes the AWt from 1 to 1.1 for a 30-rounder and 0.6 to 0.8 for a 20-rounder.