Transferring ammunition between weapons and Ammunition Storage is a vital part of trading, weapon management and teamwork. You can transfer ammo into any weapon that fires that ammo type (See Here for a list of weapons and ammo types]]
To transfer ammo from item 1 to item 2, you must have item 1 in your inventory or a container of some sort, then hold item 2, open up the item options and click the "Transfer Ammo" option, a dialog will show where you can type the amount of ammo to transfer into.
For instance, you have a tin of 5.56 rounds and an AK47 in your hands, put the tin in your bag, hold the AK47, open up the bag > tin options > "Transfer Ammo to gun". This also works the other way around, as well as with transferring ammo from one tin to another, or from one gun to another (the guns don't have to be the same type of gun, they only need to use the same ammo type)
This feature becomes invaluable when working your way up through better guns, you may pick up an M9 soon after spawning which uses 9mm rounds. And then, in a few days you find a Tec9 which also uses 9mm rounds, why waste that precious ammo just because you upgraded to a better weapon? Transfer it!