At level 38 you unlock the Overkill perk (located in the Perk 2 or red perks) which allows you to carry two primary weapons rather than a pistol. Some of the guns in online play can share ammunition. If you are a skilled sniper and prefer the bolt action to a semiauto sniper, you should share ammo between the R700 (unlocked at level 34) and the G3 assault rifle (unlocked at level 25). That way, instead of having 40 rounds, you will have 152 rounds for the sniper rifle, effectively giving you an endless supply of ammo for the R700. G3 users will gain an extra 40 rounds (two clips). This works both ways. Since you share ammunition, you might not want to get too trigger happy with the G3. Also, there are more combination so try experimenting with different guns to see which can share ammo.
Reach level 46 to obtain the M14 assault rifle and the M21 sniper. In the class that you want to have the extra ammo you will need to have the M21 and M14 as a weapon, which require the your second perk on Overkill (obtained at level 38). For the other two perks you can have whatever desired.
Create an MP5 class and select the M9 for your side arm. Since the MP5 and M9 share ammo, you will have 90 rounds of ammo instead of 60. This is recommended if you prestiged and have lost your Bandolier perk. When you reach level 32, enable Bandolier in this class to get 270 rounds of ammo instead of 180.