You should always carry an assortment of both 20rd and 30rd magazines. Both magazines offer advantages so shooters must have access to them when a situation requires each magazine's unique benefit. The 20rd magazine is the way to go if you need to get low to a surface while in the prone position, shooting off of a sandbag or on a bench. The flexibility of the 20rd mag is also seen if you are firing where there is low ground clearance such as firing when under a car or behind an obstacle. In addition, the 20rd mag allows shooters to fire prone on uneven/rutted surfaces where the ground is higher near the magazine well without silhouetting themselves to take a shot. Finally, most argue 20rd mags provide better balance and weight distribution then the 30rd mag. From my standpoint, you could always load the 30rd magazine with 20rds if weight is a concern. As a reminder, if you are using 30rd mag pouches to carry 20rd mags make sure you place a small piece of foam or other light material in the bottom of the pouch so the shorter mag does not drop to the bottom of the pouch, limiting quick access. The greatest benefit of the 30rd mag beyond the higher capacity is the ability to use the larger magazine as a monopod while in the prone greatly increases firing position stability. Pictured is my AR with a PMAG 20rd AR/M4 GEN M3 magazine that can be purchased for $14 from most online retailers. Check your favorite retailer often for inventory changes. Just like all magazines they sell fast and are subjected to a two limit per day purchase restriction. 20rd magazines are just as hard to get as 30rd magazines so if you find them buy all that you can afford.