Riflemen and Dual Gunners are nothing with out their ammo! Therefore, you must always have a stockpile somewhere.
You may use ammo that is at or below your weapon's ranking. For example, a rank B rifle may use rank B, C, D, E, or F ammo, but not higher ranks.
The Cash Shop sells 30-day ammo boxes that are used for Riflemen and Dual Gunners. They are available in rank C, B, A, and AA. These provide you with a continuous supply of ammo and does not use inventory space, just be sure you purchase the ranking you require.