To easily carry more ammo, equip two pieces of the Lone Star gear, a chest piece with extra ammo capacity as a stat, and either the Police backpack (obtained from the Reward vendor in the Base Of Operations) or a backpack with extra ammo capacity. You can also add mods with extra ammo capacity to the gear for even more ammo. Then, collect ammo from a supply crate. You can now remove all the equipment and equip your normal gear. You can do this each time you resupply to carry a lot more ammunition than you can normally carry with the gear you are using.